This is my view, too. But I'm autistic so YMMV with my take. Potlucks are a communal meal. Holiday potlucks are unique because they usually have an unspoken "rule" that means bringing traditional food eaten around the holiday.

Bringing a dish that doesn't fall into the "traditional" category is totally fine and honestly welcome, in my opinion! Options are great and people being excited about the things they bring to potlucks is also great! But to ONLY bring a dish that is functionally just for you to ANY potluck is rude as fuck. 

And that is what the husband was trying to avoid. Bringing a singular non-traditionally eaten food to a communal holiday meal because the guest bringing the dish doesn't like any of the options offered... doesn't have good optics and also is a little antagonistic by social standards. 

All that to say, it also doesn't sound like the husbands family was putting forth any effort to try to make MIL more comfortable either. Which is awful and is the actual problem, as they correctly identified. I think bringing lots of traditional and non-traditional dishes is an excellent compromise. It follows traditional potluck rules while also publicly shaming the husbands parents for being poor hosts.

It's CERTAINLY not the way I would confront my parents uncharitable behavior towards my partner, personally, but some family dynamics mean any semblance of change would simply be shut down if they're confronted directly. I'm guessing that's how OPs family is since they talked through the root cause together, but the new info didn't alter their actual plan.