OCLP on external disk, Tahoe threat on internal

Yesterday I installed Sequoia on an external NVMe disk on a 2015 iMac. I installed Open Core onto the external disk and never touched the internal SSD which has the last supported OS Monterey on it.

This morning I had a very hard time booting into Monterey but after three restarts it finally booted. Then came the surprise 😱 that Tahoe is available. How did it get to my internal disk?