Isiah Thomas won over 30 years ago, and Iverson never won. Using those guys as examples works better to prove the PG theory than it does to refute it.
Here's another way to look at it-- since 2000, these point guards have been MVPs: Iverson (some argue he's a SG), Steve Nash, Derrick Rose, Steph Curry, Russell Westbrook, James Harden. Only one of those guys has won a championship, and he's the best shooter in NBA history.
Here are the other MVPs since 2000: Shaq, Timmy, Garnett, Nowitzki, Kobe, LeBron, Durant, Giannis, Jokic. Every single guy here except Jokic has a championship, and five of them have multiple les.
Since 2000, there have been 2 finals MVPs who were PGs: Billups and Parker. The other 20 were non-PGs, and all were 6'6" or over except for D Wade.