Even if they did use unicode, any codeset , glyph or language changes over time , ulimately they emerge out of communication, not the other way round.
If some culture decides they want to use the glyph “2” to mean a word “to”, they can and will, and no codeset is going to stop them. And if they get their message to their intended audience it doesnt matter that somebody else’s isnumber fuction get’s it wrong.
A person, community or standard codeset or dictionary cannot deny the accuracy or content of encrypted communication just because they can’t decipher it.
Put another way a more robust isnumber() should maybe have a second argument to specify the codeset being used, and maybe whether written words - in some defined languare - are also to be converted