apart from Manchester United games I see every top team is not really serious with the FA Cup, only for lower-middle teams are serious about lowering the main squad and the best formation scheme. Big team defeats in the FA are commonplace and it's as if this defeat is not a big deal, In fact, this defeat gives a fortune for them because it reduces their busy schedule and gives them more time to focus on important games on league or UCL.
There is a lot of truth in your words, but this is a general situation. If we talk about yesterday's match, then United played with the first team and there is no doubt that they planned to win. But in the end, we saw another unexpected result from them and many betters got their multi bets collapsed (if I'm not mistaken, the odds of Manchester United winning were 1.3 and it is obvious that many betters chose this game because they thought that the result is guaranteed here).