While I do think there's merit to your arguments based on what you've said, I also do think a significant part of this may legitimately be due to level discrepancy. I think your average unit was about 4-5 levels higher than mine at the time of that battle, and the lowest level unit I saw you using (level 35 I think? maybe I missed something) was substantially higher than my lowest (level 27 or 28, forget which at the time of 19). Given that the recommended player character level for the chapter is 37 (which from my experience is an overestimate anyway), and it seems like most of your units are starting it either at or above that level, I do think the strength of units plays an important role. My units just weren't capable of wiping out the enemies in the nearby vicinity of other enemies like yours were...and they certainly weren't capable of taking any hits from DK, even with rallies.
Stats do matter of course, but team build and strategy moreso, atleast they should in a strategy game. You mentioned making Leonie a Great Knight earlier, so even if your units were the same level you still couldn't do the same. Really the only difference my units being 5 levels lower would make is perhaps needing to use more Brave weapons where I used Irons. Magic and Braves would have still 1 shot, and against Death Knight I'd still survive if he doubled me. The only thing I wouldn't be able to do is have Claude kill that Calvary as he wont have the speed to double with Brave Bow, but he would just suicide into Mariane/Flayn anyway.
Again I've not done auxiliary missions, only paralogues and some rare monsters, so I think my levels are normal. That thought is only reinforced after a quick look at other people's Ch.19 teams on Youtube. Perhaps you recruited more units and you're spreading experience between all of them. I just got Ashe and Bernadetta after seeing how 10 units is the usual amount you can bring into a mission.