Packt has been known to be slop even many years before AI. Very little proofreading / editing, authors rarely being well-known experts etc.

Packt? Avoid, avoid, avoid. This is true. Their stuff tends to be slop grenades thrown over the wall. And as you correctly point out, this was the case before generative AI.

My experience with Manning, OTOH, tends to be good. Most of the material I've seen from them was human-written.