From what I can gather from a cursory glance at the docs, zed uses intelephense and its diagnostics can be disabled. The whole lsp can be disabled. At the risk of sounding like I'm saying "you're holding it wrong", I have to say that this is an OP problem and not a Zed problem. These are sensible defaults that work for almost everyone, in my opinion.

There are no "defaults that work for everyone". Well designed tooling acknowledges that and makes it easy to tune the software to your preference.