@mikestern Thanks to you, I managed to dodge a bullet. For posterity, I'll post a short write-up of my findings (Jul. 10, 2024).
Long story short, I assumed that fonts included in macOS are all free to use in apps for the Apple ecosystem, but your answer prompted me to double check that assumption.
Warning: I am not a laywer, and this is not legal advice
Usage of the fonts included in macOS' FontBook are governed by the macOS Sonoma Licensing Agreement, then by the embedding restrictions and licenses set forth by the actual creators of the font.
On page 3, under Section 1:
E. Fonts. Subject to the terms and conditions of this License, you may use the fonts included with the Apple Software to display and print content while running the Apple Software; however, you may only embed fonts in content if that is permitted by the embedding restrictions accompanying the font in question. These embedding restrictions can be found in the Font Book/Preview/Show Font Info panel.
The word "embedding" is key here. In similar contexts elsewhere, the word implies non-interactive uses like in posters or a document, and typically excludes apps. It would be safer to assume this also holds true here. This means one should instead look to the font's licensing information, which can also be found by opening FontBook and opening the info panel (Cmd + I) and looking under "Usage".
Many of the fonts have restrictive licenses, and among these are the SF family of fonts. These fonts, despite being the default font used in iOS app development, cannot be included as individual fonts in the app. It would be mind-bogglingly counterproductive for Apple to place the system font under a license that disallows developers from including it in their apps, so I can safely assume that one variant can be used via the system font, just as you said
Absent actual legal advice, I would play it safe and avoid all FontBook fonts except those with a license explicitly allowing commercial use, such as the SIL Open Font License. A safer repository of fonts would be Google Fonts, where the vast majority of fonts are under SIL or Apache.