The term I've seen a lot is https://en.wikipedia.org/wiki/Unit_in_the_last_place

So I'd probably rewrite that code to first find the ulp of the larger of the abs of a and b and then assert that their difference is less than or equal to that.

Edit: Or maybe the smaller of the abs of the two, I haven't totally thought through the consequences. It might not matter, because the ulps will only differ when the numbers are significantly apart and then it doesn't matter which one you pick. Perhaps you can just always pick the first number and get its ULP.