There are n types of people in this world: Those who don't understand numeral systems, those who understand base x systems for x ∈ [2, n] and those who get pedantic about this meta-joke.
There are 10 kinds of people in this world. Those who get quaternary; those who don't; those who thought this was going to be a ternary joke; those who can see where this is going...
Regardless of what base you're using, 10 is always the nth number. In base 10 (normal numbers), 10 is 10th. In base 2 it is the 2nd.
1
10
11
In base 16 (hexadecimal) it is the 16th.
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
10
The original joke is "there are 10 kinds of people, those who understand binary and those who don't l" because 10 in binary is 2 in base 10. But they're pointing out that a similar joke works for all bases of numbers.