I hate to bring my OSX mentality into this... but I have no idea
OSX just has a set of "zoom" levels that let me make everything smaller or bigger in a very "fractional scaling-like" way (intermediate levels are between 100% and 200%) , and it works perfectly.
And it's not like just the font changes, it scales every type of UI element perfectly implemented with every type of UI toolkit
I know part of it is rendering to a frame buffer that's larger than the physical screen's resolution and scaling that, it might just be oversampling enough to not lose visual information when scaled back down.
But that's the beauty of HiDPI in OSX, I have literally never spent a single brain cycle trying to learn how it works, because for me as a user it's easy (just hit the size looks right), and for me as a developer it's seamless (just add 2x assets).
I'm not experienced in this, but isn't fractional scaling inherently blurry?