They got 'everything' done in the break? Not sure what you mean by everything. They talked to Bruno over last summer and only went for Bruno because Pogba and McTominay were out injured for majority of the season. As for Ighalo, he came in as well because Rashford was out for a very long time and Martial had minor injuries here and there which is why we had to rely on Greenwood. They didn't do everything. They did the bare minimum and the gamble paid off. We got rid of Lukaku (a proven goal scorer) and replaced him with no one until now in the form of Ighalo. Chelsea have always done well when it comes to transfers and their board is a lot better when it comes to how quickly they get the deal done.
United weren't in position to make all their changes to the squad in one window. They waited and were patient to see what would happen, if even it meant they were going to be short on bodies and quality going into the season. That was a gamble they took and it blew up in the first 3/4 months of the year. It was going to take at least two years after the 2019 summer window based on the development of current players like Rashford, Fred, Martial, McTominay. Chelsea also haven't spent any of their Hazard money, which is incremental to the money they had from not having an full transfer window. Bruno accelerated the pace at which Pereria, Lingard, and Mata don't see first XI football, especially in the league. McTominay wasn't as a big of a loss because of Fred and Matic. There was an entire table of talent gap in the attacking central midfield position as Pogba was playing deeper and when he was out, it was a glaring black hole.
Ighalo was more of an emergency signing than Bruno, who was tracked for months before he signed. If United didn't get him in the winter window, by all accounts they were after him in the summer window.
Chelsea have a lot more work to do on their squad and they made good signings too, just like Bruno and Ighalo were for United. I agree as well that Chelsea's done a really good job continuing to bring players in and out that doesn't set them back for too long or they have just enough pieces that consistently allow them to compete for Top 4, as evident by the revolving manager door.