Initially Moranis’ point was Giannis is far and away better than Tatum, that the third best among Bucks-Celtics is KM and the 5th best is JH. If that’s your perspective obviously you think Milwaukee is the better team - even if you give the edge to Boston on the remaining starters and bench.
I'd rank the players a little differently. Giannis is clearly better than Tatum. Giannis is top 3, maybe #1, in the NBA. Tatum is somewhere around 10-12. That’s a meaningful gap and if a case is to made that Milwaukee is the top team in the East the argument begins and ends there. To say with any measure of confidence that KM and JH are better than JB and MB is a reach - you can go either way but a more grounded assessment in my opinion is to say that KM and JB are pretty even for 3rd/4th and JH and MB are pretty even for 5th/6th with Rob breathing down both their necks.
Smart, Rob, Horford White, Grant, Gallo looks pretty solid to me as remaining starters and bench. I think that does make a difference in Boston’s favor. Bottom line is the teams are pretty close and I wouldn’t bet against either.
To be clear, I never said Middleton was significantly better than Brown, they are close. I do think the gap between Holiday and either Brogdon or Smart is clear, but they are close enough that it isn't significant.
Also, it isn't like Milwaukee has a terrible bench. Lopez is very good. Portis, Allen, Connaughton, Matthews, Carter, whatever they get from Ibaka, Hill, and Ingles or their 1st round pick Beauchamp. They have a very strong supporting cast around their top 3 players. They are older obviously so there can certainly be some deterioration and Lopez obviously missed a whole lot of last year, but was back for the playoffs so he is healthy now.
The gap between Giannis and Tatum is significant. Even just small increments at their levels, matter so much. Giannis is the best player in the world. He is the best defensive player in the world and a top 5 offensive player (arguably the best given totality, but if someone wanted to say Jokic, Curry, Durant, etc. I'm not going to nitpick).