I suppose it comes down to whether the Homebrew maintainer who accepts the formula PR containing the hash is actually examining the upstream code in detail. I think that is unlikely; there's too much code for Homebrew maintainers to be experts on everything contained in homebrew-core and follow every single patch.

So yes, the hash prevents the upstream project from switching out the code at any time, but if they wanted to add some malicious code all they have to do is file a homebrew-core PR and hide it in a legitimate change.