The 1-3-1 zone does lower the rebounding and inside D on his team but even with that, his rebounding was only a third of a level lower than your own rebounding and his ID was only one level lower than your IS. That means you were slightly better than him in those spots but his OD was sooooo high that it probably cut off a lot of easy shots for your big guys especially if they don't have very good handling. If you were to have even decent backups who can contribute more than 1 block and 4 rebounds in 30ish minutes, then you would have obviously won... So yes it is because you have no depth at the big man spots.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs