Sure, it parses, the same way !isNotEnabled compiles. Doesn't mean I'd want to read it in a PR. You should read the book 'Clean Code' by Uncle Bob if you haven't already.

The sentence may be valid but it has poor readability. That was my point.

It's !believes(!isSerious(mentalHealth)) which is both poor code and writing.

'I think mental health is a serious concern' would be preferred.

Both are true. It's a less than desirable structure, but it's not going to lose somebody used to writing code. You simplify the expression when you find something like this, but to simplify it you had to understand it first.