What you are saying is all correct, but I think we disagree about the terminology. You said "the type system can never know what is semantically correct" but I think the right way of saying it is "the type system can sometimes not know what is semantically correct".
Because in practice, code like "src := fromAccount.toSourceAccount()" is very rare and happens at system boundaries or logic boundaries once - after that, all semantical errors can be caught by the typesystem, and this accounts for the vast majority of the code.
So yes, the typesystem can and will prevent semantical errors. But it can not protect you from making them in all cases, that's impossible.
Because in practice, code like "src := fromAccount.toSourceAccount()" is very rare and happens at system boundaries or logic boundaries once - after that, all semantical errors can be caught by the typesystem, and this accounts for the vast majority of the code.
So yes, the typesystem can and will prevent semantical errors. But it can not protect you from making them in all cases, that's impossible.