If you do not disconnect the loop the main battery can come online and supply power to the 12v system. All the how to battery replacement explanations, including ohmmu, state to discount the loop. There is no harm in disconnecting the loop.
Ohmmu doesn't talk about the high voltage loop for the model 3 in their
install manual; they say disconnect the negative of the 12V lead acid, and then disconnect the gray connector on top of the penthouse under the rear seat. (Granted, disconnecting the gray connector might be accomplishing the same thing as the first responder loop)
This matches the instructions from the official 2019 model 3 service manual for 12V power disconnection:
1. Disconnect any charging cable.
2. On the touchscreen, power off the climate control system, and wait at least 30 seconds for the climate control system to completely shut down.
3. Make sure the vehicle is in Park.
4. Fully lower all door windows to avoid glass shatter and door lockout conditions.
5. Open the LH and RH rear doors, and then pass a shop towel halfway through each door striker to prevent the latch from securing.
6. Remove the 2nd row lower seat cushion.
7. Remove the rear underhood apron.
8. On the touchscreen, touch Controls > Safety & Security > Vehicle Power > Power Off.
9. Loosen the nut that attaches the 12V negative cable to the battery, and then disconnect the cable from the battery.
10. Remove foam cover from the RH side of the penthouse.
11. Disconnect the electrical harness from the high voltage controller connector. (the HV battery positive contractor and negative contractor open with a clunk sound).
That all being said, my original question was how these above steps change for 2022 Model 3 (and model Ys) with the new 12V lithium battery. My guess is it's just that step 9 is replaced with something like disconnect the 12V lithium battery connector. But I wanted to know if anyone had additional insight.
