Bitcoin Core Pre-Instructions - Is This a Security Trade Off?

The way that assume valid works, and please correct me if I'm wrong, I'm not a Bitcoin core dev. Whenever the Bitcoin core developers cut a new release of Bitcoin core, there's this block hash that is hard coded into the code. And it's meant to speed up IBD, right? So you don't have to validate everything before that point. Well, you validate a lot of things, but so you don't need to do signature checking primarily. You still validate that the work is in the headers, that the transaction puts output match, all that stuff. All you do is you just skip the witnesses. Okay. Or the script... Or the script SIGs,