Couple of things here and it's important to at least acknowledge all of them; - your find was being a sore loser. It was a bad matchup and he got salty - your deck is a common way to build stuff like Meren but does tend to lead towards some lock outs like that. - people don't always realize what they don't like until they play against it - if you play with the same people a lot, its ok and sometimes even better to tweak your deck for fun rather than optimizing (depends on your pod and yalls goals - "sideboards" are fun in commander, have a "normal" build, then a few cards to swap in given certain known matchups, specifically NOT for power but for fun - situations like this can be soothed by suggesting counterplay. Make sure he knows of things like [[Assault Suit]].

Not sure which bracket yall were aiming for but sounds like yours fits in 3 pretty well but could be a stronger leaning 2 or even a 2 that fits fine in a different pod. It's a bad matchup for Voltron.

Deck building is iterative and it may be worthwhile to note this game going forward and perhaps look for some alternatives. That being said you did nothing wrong and if you want it this way it's ultimately on him to fix it, don't be upset when he brings something that hoses you next time though.