We don’t actually know if the champions fought Murlocs as lore wise there are multiple adventures some could have been fighter murlocs with no artifacts while others could have been fighting kil’jaden with the artifacts.
We know a few things, boss kills have been broken up into factions lines pre argent tournament so not every boss has been killed by every adventure. The tournament had the best of the best join forces to beat the lich king.
The horde and alliance then worked together for cata-mop meaning the same adventures from the tournament could have delt with all the main bosses while lesser characters were doing other things. Then in legion the best of the adventure got the artifacts and lead order halls this leaves lesser Murloc killing folk to be part of said order halls. From there we know that said leaders got the heart of Azeroth.
In short lore wise all we actually know is that we killed the main bads in legion then those same characters lead into bfa we don’t know if every single quest was done by the order hall leader or lesser adventures all we know is we did the artifact/main story ones.