"expired" doesn't mean you can't sue for infringement. For six years (someone check me on this), you can sue for infringements that happened while the patent was unexpired.

I seem to recall that you have to show intent to commercialize a patent in order to enforce it. Is that a thing, or just a bit of urban folklore that I picked up somewhere?

IANAL but I don't think that's true.

A patent doesn't allow you to practice an invention; it allows you to exclude others from practicing it.

If I understand correctly not applicable here (patent expired after max duration of 20 years), but one facet of this is that in some cases you can buy a patent that is expired because the owner decided it wasn't worth to continue pay the extension fees and un-expire it by paying the fees retroactively.

I find it super bizarre that Huawei would sell expired patents. Is it really worth it?

Are you asking if it's worth it to them to sell something that is worthless to them?