I don't think very much should be read into that Man U victory over Liverpool. Yes, 4-0 sounds quite comprehensive, but it has to be taken in context. It's not a competitive match, and not only is it pre-season, it's early pre-season and players are still getting fit. But the biggest reason is the players they put out, Liverpool made so many substitutions, they had over 30 different players appearing in total.
https://www.bbc.co.uk/sport/football/62141591In this case, apart from heating up the engine, the players are here to see which formations and schemes are suitable.
On the other hand, it is quite natural in this case to replace almost all of the players there because it is like a test to see how the players on the field are too.
Actually it looks good to judge each other between the two clubs but it seems Liverpool only take this as a fun match.