I had the same reaction. This seems like the plan of scanning of pictures on iPhones for CSAM; it would not be hard to add extra patterns that match materials beyond the original intent.

Are the secret patterns all publicly available? Or is the secret scanning patterns themselves secret? Without public review, we cannot know what secrets they will obtain.

I for one do not trust GutHub/Microsoft to act in the interest of the average user. Their past actions disqualify them from receiving any benefit of doubt.