Yes, I know. The court at the O2 arena is terrible for Nadal's game. And yes, he would probably dictate most of the rallies against Zverev at any of the Slams (except maybe Wimbledon if Zverev was serving really well). But time after time we have seen Nadal's forehand not just look pedestrian on indoor hardcourts (particularly World Tour Finals) but even ineffectual.
Now when we look at other players regarded as having great forehands, they of course have their venues where their forehand is better than at others. Federer's forehand, for example, is a nuclear weapon at Wimbledon but not nearly as dominant at Roland Garros. But does Federer have any events on the calendar where his forehand is nearly impotent like Nadal does? Nadal is lucky to get 5 winners in a match with his forehand at the O2 arena.
Today, Zverev hit 14 forehand winners to Nadal's 3 (Nadal had 5 BH winners to Zverev's 3, and Zverev had 11 aces to Nadal's 5 for a total of 28 winners to 13 winners). Does it really make sense to say that someone has the best forehand in the game when at the year-end championships they walk on court after getting the #1 ranking and can't even hit 5 winners in a 2-set match while the opponents hits 14? What kind of "best forehand" are we really talking about in that case?