Suffice it to say that since 1988 when I got my first Sinsemilla Tips magazine with a lengthy article on PM and other nasty molds, I have seen a multitude of different cures and fixes.
Not one in 23 Years+ mentioned using pH Up as a solution or fix. If it actually worked and didn't kill the plant in the process, I'm sure it would have come to light by now, stoners being the experimental souls they are.
I would guess "it will kill the plant" is the most obvious reason, IDK. Perhaps it just doesn't work. Again, IDK. I don't really care either. Its omission, all these years is answer enough for me, and for sure, you aren't the first person to think of this.
Give it a shot and let us know how it works out.
Sorry if I sounded unduly snarky, it happens.
Wet