The answer is No, it does not downsample to 1080p displays.
In fact, there is a theory going around that the Pro visual modes are all bugged for 1080p users, including HFR.
One person claims that the game runs and looks identical on their Pro and HG PS4 hooked up to the same 1080p TV.
I obviously can't vouch for that myself, I don't have an OG PS4 anymore, but I find it very very odd that with a 1080p display I can force any of the visual modes even after it warns you it's not supported, and not notice a difference at all between any of the 3. HFR, Enhanced, and 4k all look and play identically on a 1080p display, and honestly on my 1080p display I don't even feel like HFR, the one mode we are supposed to get at 1080p, is even working. I get all the familiar drops in cutscenes and parts of RTTS games as before....
Very very odd. Something isn't right