Right, but "slur" by itself is used as shorthand for "offensive slur" - typically racially-offensive slurs or homophobic slurs, and so on - I'm not aware of normal public discourse referring to everyday insults as "slurs" - whatever the original definition slur had it now has a connotation of insulting the target by way of comparing the target to an intentionally hurtful stereotype. (I could enumerate examples but nothing good can come from that, methinks...)

Whereas "TERF" is an acronym - it isn't appealing to anyone's emotional opinions about what a "TERF" represents - that is if anyone is even clued-up enough to know what it actually means.

Maybe the people complaining are assuming that TERF is an offensive slur without doing their research first? I'll admit it does have that feel to it, the way it sharply rolls off the tongue...