@DuckieTigger: There aren't multiple versions (different sets of ttf files) of built-in font for each level of boldness, I've checked that squashfs image containing those font files. So it's probably done "on-demand"... logs contained messages mentioning "embolden" and increment steps of 20 (20,40,60,80,100)
I thought of that possibility when I've noticed that bold settings have no affect even if you override one built-in font with another built-in font.
As for font override + KFX format: I've noticed that with latest version of fonts hack and PW3 on firmware 5.6.5 my font override (I'm using edited Bookerly with reduced descender/ascender metrics for smaller line-height; I've renamed font family to "BookerlyMod") was not working for KFX books. It displayed regular Bookerly when I've chosen Caecilia Condensed (overridden to my BookerlyMod). And KF8 books worked fine.
Now on firmware 5.8.11 I've tested all that again and it looks like override is ignored and it display Caecilia Condensed
