Despite owning Klopp's squad legacy, but Slot knew better about United rather than ten hag. If you're watching Slot's post interview, then you would see how he was breaking down Ten Hag's pressing. he even exposed United's tactic to the public.
When you see an experienced manager who knows how to study the tactical structure of his opponents before playing against so he can device a means of coming top at the end of the day, you'll definitely not struggle to notice about his professionalism. Arne Slot came to Liverpool and started with by studying the tactical approaches of his predecessor Jurgen Klopp which was why he was able to use the squad at his disposal effectively without even adding a new player in their mix. Like you rightly said, he exposed the weakness of Erik ten Hag's tactics which was why deviced the best tactical approach to get all three points from the game at Old Trafford yesterday and that actually worked perfectly for him.
Knowing the strategy of an opposite opponent is common and understandable, starting a working process is actually hard and I must give Arne Slot an accolades because not every new coach can start the season newly with an amazing performance, when Arteta started also he got some similar issue even few old coach find it difficult coping this season.
Talking about Ten hag strategy is kinda funny since few people can tell their weakness and strategy although that’s just normal, last season when Arteta and pep Guardiola played their last match it got complicated mainly because they knew themselves, also Unai Emery knows how to play against Arsenal so it’s normal. Ten hag and his team just need to sit up if they want a chance or else their fans should not expect something serious this season.