It's running the same compilation steps that the binaries are produced from. If you can get a binary through homebrew, you can compile it from source. It's just unsupported in the sense that the developers don't want to spend their time chasing down build issues on individual user's systems. You're on your own if you break your local build environment.

(Ignoring the packages that are just wrappers around closed source software and fetch binaries whether you compile from source or not. Those are the exception, not the rule.)