triplex to a detached garage (3 wire ?)

[kwired]

So, if it is a “defined garage” you need two circuits minimum.
But you may only run one circuit (or mwbc) without running a feeder/sub panel.
Hence, if you don’t want to install a sub panel, a mwbc is your only option. Is that correct?

Probably exactly why they didn't change other code sections when they made garages require more than one circuit, not all outbuildings are "garages" and you could still run a single (two wire) circuit to buildings that only require one circuit.