How can a Pro machine use integrated graphics?
Because not every pro needs a GPU. Gaming is not a profession despite what some people might think. Those that are gaming professionals don't earn money by how good the game looks on their computer. They earn money from speed running and being a good gamer. Frankly a pro gamer buying a Mac seems a little off to me anyway. Even if they got killer specs they still need to fart around with bootcamp and setting up Windows 10 on their own. With the added cost and that extra effort I just don't see the point. The only gaming aspect is for those that want a Mac but happen to occasional game and still want to be able to do so. Those typically are not the gaming pros that earn money from it however.
As for things like animation and video production that's a tiny market and you can still do those things. FCPX surprisingly works very well on the 13" MBP. Sure it takes much longer to render effects but then not every pro uses a lot of effects. Most Hollywood movies could totally be cut on a 13" MBP that are cuts only with a bit of titles here and there.
I also want a 13" with a dedicated GPU but a dedicated GPU absolutely does not make a MBP pro or not pro on its own.
There are professional authors, illustrators, graphic designers, photographers, vloggers, teachers, scientists, politicians, business executives, sales people, software developers and so forth that may never even touch a dedicated GPU.