It is in one sense and isn’t in another. Someone saying “basic biology” is generally ignorant of the wide world of biology out there, either through lack of exposure or purposefully so.
The truth is that biology’s definitions are all humans’ ways of dividing up phenomena into neat little categories, but nature just exists as it is and doesn’t need to follow our linguistic rules.
For every definition we come up with, there are invariably multiple exceptions that don’t cleanly fit, because that’s just not how nature works. Even basic definitions like “sex,” “species,” and “life” itself start to get shaky the moment we try to eliminate all the exceptions.
The truth is that for humans there is no single, universally accepted definition of sex or gender. And even attempts to reduce them to something tangentially related like genotype quickly fall apart when you start looking at the exceptions. The person who says that there being two human sexes (usually to the exclusion of gender identity) is “basic biology” is not only categorically wrong, but they’re either ignorant of or ignoring the granularity of our own species. The whole of human sex and gender identity cannot be neatly summed up as “Boys have a penis, girls have a vagina” from Kindergarten Cop.
So are the slugs relevant? I would say yes, because if you’re not aware of the variation in nature on the obvious macro scale you have no prayer of appreciating it on the more subtle micro scale.
The silver lining to the shit show of the last 9 months is that the Republican drama has finally gotten entertaining.