Assuming they use the same numbers then yes, 10 is 10 to us but it represents the separator of orders of magnitude. 10 in base 4 would from our perspective be 4, but ignoring the specific numbers and adapting them for the sake of the joke, the alien would write it as 10 and “base 10” would just be base 4. It’s like how hexadecimal uses 0-9 and a-f to represent 16 numbers even though we don’t have that many in decimal systems. The joke is a translation issue. I feel like I’m over explaining now though…