I think that there are benefits to not exposing the AST in a compiler, for example the freedom to radically change its internal structure from one release to an

I think that there are benefits to not exposing the AST in a compiler, for example the freedom to radically change its internal structure from one release to another without having to maintain backward compatibility with software that use the now public API. He could have used this argument instead, having a public API will put more work on the developers that now have to maintain it. But the argument that this will make it easier for proprietary software to use GCC to parse code doesn't make any sense.