He's right.
There was an amazing season, going down to the shootout in the final race and Hamilton won. He beat him on the track and it was taken away by an incompetent official breaking the rules to give Verstappen the win.
Everyone knows that. The fans, the media and the drivers. And now as the hysteria wears off Verstappen will know it too.
And when Masi is fired, and when regulations are changed because of it, its going to be cemented in history. Every time the new regs are referenced, or the new race director, they're going to bring up the time the FIA stole a title from a driver. Every time Hamilton is referred to as the most successful driver of all time with 7or 8 or 9 titles wherever he ends up, people will mention he should have 1 more, if not for the FIA cheating. It will go on for decades.
The only reason anyone is accepting this is because Hamilton has 7 WDCs already, which is ridiculous, but there we are.