I agree with your sentiment but Dallas being matched up with +50 win teams the entire playoffs slaying the best in a superior West while the Celtics have been facing hospital wards is a major reason I'm picking the Mavs. Battle tested.
I guess it depends on what people consider "battle tested." Some of the core guys on the Celtics have a ton of playoff experience since their initial run starting in 2017. Since then, Boston in the playoffs has beaten Embiid 3 times, Giannis and Butler twice, as well as beating KD and Kyrie. They also played LeBron twice and Steph once. In that time, they've won 15 series. Does that not qualify as being battle tested?
Of the Top 20 guys in the series, here's the breakdown of playoff experience:
Horford - 181 games, Brown - 119, Tatum - 108, Holiday - 84, White - 67, Pritchard - 53, Hauser - 36, Kornet - 29, Tillman - 24, Porzingis - 14
Kyrie - 91, Doncic - 45, Jones - 39, Kleber - 39, Hardaway - 38, Green - 34, Exum - 33, Gafford - 22, Washington - 17, Lively - 16
Record the past 3 seasons vs. teams with 50+ wins (regular and post season): BOS 42-34, DAL 37-44. Yes, the Mavs have beaten three very good teams to get here (including two with hardly and playoff experience). Oddly enough, there's an argument that BOSTON is the more battle tested team looking at a full body of work and not just the last month and a half.