I thoroughly enjoyed the ATP podcast discussion on this.
Sounds like the base assumption is Apple is incapable of having decent tethering even as they control the whole ecosystem, including special protocols in-between their product they design 100% from hardware to software.
But somehow pushing the modem in the MacBook solves this...because Apple is then good at managing transient network connections in macos, hooking to complex mesh networks on protocols they don't control ?
To my eyes Apple being unable to realize these dreamlike expectations is probably the very reason MacBooks don't have modems.