OCLP on external disk, Tahoe threat on internal

Yesterday I installed Sequoia on an external NVMe disk on a 2015 iMac. It was a challenge but I got it working. The purpose is to test it and not disrupt the working system. I installed Open Core onto the external disk and never touched the internal SSD on which the last supported OS Monterey is installed.

This morning I had a very difficult time booting into Monterey but after three restarts I finally got to the desktop and to my surprise 😱 my Monterey SSD is upgradable to Tahoe!

How did the Tahoe upgrade get to my internal disk?