Then your solution seems to be the obvious one - if he's GT and he's in an accelerated math class, then it makes perfect sense that he should receive enrichment at that grade level.
I often find that the play-dumb email helps here. "I don't get it. DS is in 5th grade math, and he's identified as gifted. Can you please explain to me why he shouldn't receive enrichment at the 5th grade level?"
With that said, kids often enjoy enrichment activities, and it makes the school week more bearable, but I don't often find that they're learning critical skills in there (I'd love to hear examples of the opposite). Given that they're already open to fixing this in a couple of months, I wouldn't necessarily make this my hill to die on. It might be worth saving some of your political capital to spend on other things later.
