But doesn't this tax only effect very highly compensated employees
No, and that's the problem. Your employer typically pays around $10k to subsidize your health insurance, regardless of what your pay rate is. This proposal is much closer to a flat tax, the same number of dollars from every person, than it is to income tax, where the highly compensated pay more. Republicans love flat taxes, because they are so regressive. They are better for rich people.
And as an added bonus, this proposal is popular with Republicans because it disproportionately hurts unions (teachers, firefighters, police, civil servants, and factory workers) because those groups have negotiated lower pay in exchange for better insurance. If you start taxing that insurance, it's like selectively taxing those union members. Republicans love the idea of selectively taxing only union members, because unions typically support Democratic candidates.
So once again, it's all about party. The robots thread had largely agreed that our economy is stagnating as a result of Republicans "pro business" policies, which favor the wealthy and hurt the American workers who provide the demand that really drives our economy. Their new ACA repeal plan is just more of the same, another way to enrich the wealthy white constituents of the Republican party while screwing over America.
Of course, they don't see it that way. Republicans think the "real" America is just their wealthy white constituents, not the vast middle class. I suspect they honestly believe they are strengthening America by screwing over most Americans.