https://developer.garmin.com/connect-iq/core-topics/graphics/
The clear implication is that scalable/vector fonts are system fonts only, and not custom fonts.
Ofc the docs could be out of date. I'll just say I haven't seen any evidence that a custom font can be a vector font at any point, but I have seen evidence to the contrary (docs, forum posts, etc.)
I would think if they added that capability at some point, it would at least merit a mention in a CIQ announcement.
Would be a nice feature tho.