The point I'm contesting is that it's more difficult to validate on the server using HTMX.

And I'm not saying you should be only validating on submit when using HTMX.

Client exclusive validation is quite limited. Very often you need a trip to the server anyway so I don't buy the latency argument.

Plus in most cases showing error messages too fast is terrible UX. The only exception I can think of is when checking the validity of a password.