Their squad depth is not as good as before - especially in defence. Savinho is not very good as a winger - while City also lack balance in midfield without a good defensive midfielder. Pep Guardiola can't force De Bruyne to perform like he used to - Foden isn't at his best either. Haaland was less than optimal due to a lack of creativity on the wings and midfield - so everything changed compared to the previous season.
In addition to the decline in performance, Pep's strategy can actually be read perfectly by Arni Slot. He understands very well how Pep implements the strategy, especially on 1st half. As we can see how the 1st goal created, Mohammed Salah was unguarded at the moment, even Salah is a top scorer but he is not separated from the defender's observation. It's the same on 2nd goal, the city defender repeats the same thing, Salah, and Szoboszlai still missed their surveillance, and create a nice collaboration on front for 2nd goal. That strange strategy Arni implemented 4-2-2-2, this making mess The city player when Luiz Diaz, and Jones looming them on the other side.
I don't think it was about the strategy. In fact, Pep did use inverted RB tonight. Their main problem is that in how long them to do transition. This creates a space for your opponent when only focusing on defending, then counter attacking.
I often saw how Pep always struggled against a parking bus strategy like what his club did against Liverpool. This has to be the most important puzzle he needs to solve. Once he can solve it, i believe his club may back to the winning the title when he settle it all.