The parent commenter does not suggest that this syntax was chosen based on assuming users don't have syntax highlighting, only that they did take into consideration that not all users have syntax highlighting.

I understand that, but how much into consideration? Let's just make up some numbers, if 1% of reading of Rust code takes place in environments without code highlight, then it should have been 1% of the consideration. Is that the case?

I don't think you should weight it like that. If you have multiple proposed syntaxes, and both are equally effective with code highlighting but one is moderately ineffective without highlighting, then it makes sense that you should go for the latter. The weighting controls how much of an effectivity loss in the code highlighting case you would be willing to trade off against effectivity improvements in the other case.

It sounds like you're agreeing with me. If two options are indeed essentially equal on all fronts, then one minor use case can be enough to push it towards a winner.