The issue is sapience, not sentience. To imply that even a stupid human, a sapient being, is in any way comparable to the intelligence of a dog is offensive
I'm saying they're capable of minimal animal-level intelligence, not human-like sapience implying higher thought. I believe this to be fact, not subjective opinion.
Other animal's intelligence is clearly different form human's, but every study they do on the intelligence of animals shows them to be more intelligent and closer to us than was previously thought. You assuming to understand the experience of dogs is, like you said, belief. You should have just stopped there though, because your belief is just that, a belief, not a fact.
I've met dogs that I'm sure are more emotionally intelligent than you are. Just because they can't talk and don't have thumbs doesn't mean they don't have thoughts or feelings.
I see the comic as an attempt to translate the existential stress a dog "feels" to the human experience, especially it's intensity.
Because even with no language, no consciousness as humans have it, dogs do experience intensity you could measure in cortisol levels, heartbeat, eye movement etc.
The comic is useful for those who are interested in translating that to human experience. A communicative form that works well is narrative framing. It gives your empathy a correspondant in your conscious thinking.
I like dogs. I dislike anthropomorphizing pets and unduly imbuing them with humanity. I take issue with modern dog culture that does just that, teaching people that dogs are "family" rather than encouraging normal and healthy human-pet relationships. You can and should love your dog, but like a dog, not like it was your human child.
I don't support cruelty to dogs. I think it's fucked up to have them chained up alone outside, to leave in a hot car, any of that. I just also think it's fucked up to carry them around in your purse in public settings were dogs are clearly unwelcomed. I think when you accept the responsibility of having a pet you're accepting the responsibility of making sure all that pet's needs, including for attention and affection, are met. This is healthy approach to dog ownership.
You're referencing extremes while trying to deflate the average. Most people do not carry their dogs in a purse. My pets are my family. I do not carry them around in a purse.