Just to attempt to attack this issue.
AGM batteries should never be "quick charged". 2 amps maximum current for most situations. Standard lead acid battery chargers are often 5-15 amps current.
Most likely the positive terminal under the hood is isolated, or at the very least switched. This terminal most likely allows the starter generator to see power, but isolates the AGM battery from seeing any current.
The most expected reasoning is that the relays for ignition, and bcm/pcm/ipu etc... all run on 12v supply. The 12v battery most likely does not actually do any "starting", as the Generator/Starter unit is High Voltage, and powered by the cars HV Battery Pack.
Using an AGM battery to jump start a car will most likely damage the AGM battery, unless it's specifically designed for the current output of a traditional car starter (400+ cca). While most AGM batteries in this size category can be rated for 450 CCA, the fact that our cars don't have a traditional alternator may be the Achilles heel. It's probable that the 12v charging system isn't designed for any significant load.
Or the fact that we have a remote positive, that I speculate is switched/isolated from the AGM, it could damage that terminal to jump start another car and push that kind of amperage.