I don't write Ruby regularly either, but I wouldn't say that syntactic complexity, is necessarily equivalent to semantic complexity. And the syntax is the only part that's relevant to Tree-sitter: it's not an interpreter/compiler.

Note also that (as I alluded to above) the parsing technique that Tree-sitter uses, "LR parsing", makes some things more difficult to parse than they'd be with another kind of parser. This is a deliberate trade-off, because LR parsing makes certain features of Tree-sitter, like fast re-parsing in response to input changes, much much easier.