This is an amazing new feature and so glad it has been added.
Hi. Adding this feature to SPPB is a populism for the sake of newcomers to the Web industry. And has nothing to do with the right work with the site in the aspect of performance. Professionals would prefer to have this at the template level but not inside SPPB. In my opinion, any possibilities of choosing fonts should be completely removed from SPPB. But this would blow up public opinion. :)
Helix needs this feature, not SPPB.
(IMO)
I cannot choose the font for a section heading within SPPB; instead, it pulls the font from the template
Completely turn off this in Helix to get the opportunity to use your font in SPPB. Since the setting of fonts in Helix is the highest priority (as it should be).

I hope this is what will help you understand that the fonts should be used at the global level, but not inside a separate component as SPPB. Since usually there are other pages from other components for which typography is also required. Website should use a maximum of 3 fonts (goob point is no more than 2 fonts), based on performance and good tone in design.
Therefore, with all the charms of this feature, I consider it not suitable for use. Unless in rare cases when you make a landing page and there is nothing else on the site (things like Header, Footer, articles and ect).
I apologize for my spoon of tar. :)