BigQuery's behavior is correct with respect to the RE2 syntax documentation. (No surprise, because BigQuery uses RE2 to implement regexps.)
RE2's character classes are:
\b = at word boundary (\w on one side and \W, \A, or \z on the other)
\w = word characters (≡ [0-9A-Za-z_])
\W = not word characters (≡ [^0-9A-Za-z_])
\A = beginning of text
\z = end of text
In other words, you can only use \b to match boundaries of non-accented characters. RE2 has plenty of support for Unicode characters, though, so you can most likely craft an alternative regexp using something like \pL.
I'm not sure why Google Apps Script doesn't follow the RE2 spec here, but I'll follow up with that team to figure out what's going on.