‘Muslim’ is slang for Pakistani here? I must have missed that memo.
It’s true that ‘Asian’ is very roughly synonymous with ‘Pakistani or Indian’ in common UK parlance (maybe that’s what the OP was thinking of?), but as far as I can tell, when people say "Muslim’, they’re talking about people with some sort of adherence to Islam (or maybe in some cases, people who just have some social and cultural trappings that have some sort of Islamic influence).