That isn't a shibboleth because it doesn't mark you as a member of their group. It sounds like a simple preference against gendered language (or maybe the default masculine, idk).

Using the word 'head' to refer to the bathroom would be a shibboleth.

Side note: I find that preference odd because I come from a context where 'you guys' is neutral, but ultimately view it as a deficiency in the english language.