Your conclusion is right but your explanation is so wrong...
Mac computers do not care if the headphones are TRS or TRRS. iOS devices do but not Macs Except that older Macs may not be able to use TRRS headphones.
Newer Macs do not have a mic input. If using the 'phones output, GarageBand must have access to the internal mic so that headphone output can work. It’s similar to your setup.
The key is that GB must have an input and output. It can be an external interface set up to 44.1kHz. GB does not support any other sampling rate. This also means it cannot work with Zoom which supports 48kHz only.