Let's say there is an architect and he also owns a construction company. This architect, then designs a building and gets it built from of his employees and contractors.

In such cases the person says, I have built this building. People who found companies, say they have built companies. It's commonly accepted in our society.

So even if Claude built for it for GP, as long as GP designed it, paid for tools (Claude) to build it, also tested it to make sure that it works, I personally think, he has right to say he has built it.

If you don't like it, you are not required to use it.