This is just false. UTS#18 specifies multiple levels of Unicode support. \X is part of level 2. It is perfectly valid to generally say "has Unicode support" even if it's just Level 1, assuming you document somewhere what precisely is supported.

For example, I regularly say that Rust's regex crate has Unicode support. But it does not support \X. It's more precisely documented here: https://github.com/rust-lang/regex/blob/master/UNICODE.md