> What’s hard is (...) [building] [code] under the constraints of these rules

Good point - I have to admit that, for example, the single-mutable-reference rule in Rust did force me to keep restructuring my code differently than I used to in other PLs and I did struggle with it to an extent.

It doesn't automatically mean, however, that Rust is (literally) "harder to learn than other PLs". It is possibly the question of whether "a programming concept is hard to learn since it cannot be painlessly brought over from previous experience" or "a programming concept is hard to learn since it is hard to understand in itself".