I believe that because he said it. He said his beliefs would not interfere with the Constitution. There’s not going to be a candidate who meets every single litmus test purists come up with. If they believe in the Constitution, that’s a win.
There are two things in government. that which is said, and that which is done. "If you like your plan you can keep your plan." as just one example. Personally I could never bring myself to support an anti-gunner no matter what his stance on other issues.