This is true. If the recipe says sift first, do it or you'll have too much flour. Otherwise most assume you'll scoop, scrape, pour. If I think my flour may have settled too much I might turn my canister upside down once and back upright before scooping.
We tested this in our kitchen. A glass pyrex used as precisely as possible was off by more than 5% in repeated tests. Our kitchen scale was off by less than 1% for weights over 5g.
And honestly, I am comfortable just pouring the milk/water/vanilla directly into the bowl that is on the scale. No utensil to get dirty. I recognize that I could over pour and mess things up but it just doesn’t happen. I can hit 15g of vanilla more accurately with the scale than with a measuring spoon.
It may impact you, but the same would happen climbing and having different boiling points. It may be extreme, but we are talking about convincing folks who use a spoon as a standard.
Tablespoons and tea spoons are fine as a measurement because they are made to a standard size. That's like complaining people are using a piece of tape as a measurement. That's what i grew up with, and it is what i am comfortable with. I'm not saying it's better, I absolutely agree that variables in cooking such as elevation and ambient temperature/humidity matter way more and the overwhelming majority of the population wont notice a difference if you measure by volume, by weight, or are so experienced you just eyeball it.
The difference of expansion is so little at room temperature +- 10°C that it makes less sense, especially because some liquids have varying density by default and the same temperature.
Its not necessary to be accurate to the degree you want.
I had my wife try to measure water in a glass measuring cup accurately and consistently. I had her measure the same amount multiple times. Her variance was so far off the variance of the scale, that I convinced her that liquids should be done by weight when possible.
I think that if I had a cylinder like I used decades ago in chemistry class, I might be able to get consistent kitchen measurements. But my glass pyrex measuring cup with numbers on the side is terrible.
If I make a recipe multiple times, it gets re-written for weight versus volume.
If anything, measuring by weight only is better for liquids other than water. You try measuring other liquids by volume, you run into issues with it not matching as well. One container of milk may have more or less grams per liter than another. Maybe only a gram difference, but still.
Besides, a cup is always a cup the same way a liter is always a liter. A pound is a pound. You might run into crappy measuring devices that aren't accurate, but the units themselves are standardized.
Metric makes some things easier, but other things harder.
Metric is essentially a blend of two things; a set of units (meters, liters, etc) and dividing them by decimals.
Where both the units and the decimal divisions have a problem is when you don't have well labeled measuring devices. Anyone can figure out a third, or a half or a quarter, and then divide down from there with any given container. Dividing a container into tenths, and then more tenths isn't as viable. Not impossible, just not as easy as fractional divisions.
Then there's the units. To get a cubic centimeter, you first have to take a meter and break it down.
This applies to weights and volumes as well. Grams are such a small base unit that estimating with it is difficult. Liters are the exception to that, you can easily visually estimate a liter or a gallon, and if it's a liquid that's familiar, do so by feel as well.
But milliliters, cubic centimeters, those run into trouble.
Which is whatever. But the point is that the units are arbitrary. You could take a foot and divide it into ten and have a unit just as useful for decimal maths as centimeters. It's decimal vs fractional calculations that most people bitch about. And fractional is just as useful, and no harder to do on the fly once you've gotten to about the 6th grade.
All of that may seem moot when you're baking at home and have your scale and measuring devices of choice. But when you aren't in a kitchen set up for your preferences, relying on the smaller units becomes a problem. If you're away from an actual kitchen, the gap becomes even more significant because it's not that hard to make containers that approximate cups with accuracy, less so with liters, and even less with milliliters.
That's because most of the cooking units in imperial pretty directly fit readily available objects, with a small enough variance to not waste resources in the process.
Now, you can learn to visually or tactilely approximate SI units too. You can get used to the weight of what a hundred grams feels like, or what it looks like when it's known material. It's just not as simple as the imperial units in that regard.
Now, if you want to run into where imperial units really suck, look into the different standards for what sets dry and liquid measure by volume. They're different, but a gallon and a cup are rarely specified as liquid or dry, which means that the other units derived from them are a pain in the ass if you don't know the difference. And don't get me started with the fact that us standard units and imperial units aren't actually the same in every case.
But, again as units they are standardized and no more or less useful than SI units inherently. Nor is fractional vs metric/decimal a comparison that's inherently better on one side or the other for all uses in the kitchen (and definitely not in the general sense).