High level the idea of the false 9 is to provide numerical advantage in possession through the midfield, create triangles in attacking areas & tactically it presents a dilemma for CBs with being pulled out of position. The latter part of this happens no matter what formation you play in your midfield but, by playing Fred/McT in a double pivot I feel the onus is way too defensive and cedes too much space to KDB/Gundogan. Pep used to play us with an inverted triangle with Fernandinho or Rodri as the sole DM, then two more advanced midfielders whereas the last two times he has played us, after Ole's great record against them last season, he went with a double pivot of his own in that awful 0-0 draw and last night used a false 9. On a very basic level, think of the strength of a double pivot and how it matches up to that traditional formation, it is a lot easier to mark players (you can have one CB pick up the No9 and one sit off) and track runners and is why Pep is usually reliant on attacking full backs to create overlaps/extra passing options as most teams go defensive and sit deep against him.
AWB-----Lindelof-----Maguire-----Shaw
Mahrez-------------- Aguero -------------Sterling
------------- Fred ------- McT--------------
---------------Foden --------------KDB ----------------
--------------------Bruno -------------------
------------------------ Fernandinho--------------------
I personally think, as evidenced by his tactics in the 0-0 draw, Pep knows our only threat is if he has his full backs bomb on and Martial/Rashford can run into that space and run at one of the CBs or we get lucky with a long ball/error. He can't really change the last point but he can mitigate the former. Therefore, if he's not letting Zinchenko & Cancelo both bomb on at will (Cancelo kept Pogba wide but he never really got that far forward, same with Zinchenko and Rashford) he needs to create an overload somewhere and that's what the false 9 did. They had a fluid front line but, again high level, this is how they created so much space in my opinion as Mahrez/Sterling would drop off and it basically meant our midfield ran itself into the ground having to try and cover the extra man and the absolute chasm of space that is between Bruno and the double pivot, there's a reason people kept saying McT and Fred were anonymous because they were just constantly trying to cover ground and shut down passing lanes. That formation contributed heavily to City looking so good last night because 1st half I think people would agree we were in the game, contesting the ball and doing well but that level of work rate for Fred/McT is unsustainable for 90mins.
Maybe a better way to phrase it would be that specifically playing Fred/McT as the double pivot was made redundant by them using a false nine because, that is to say two technically limited but combative CMs was an overly cautious decision from Ole in my mind. If Ole is set on using the double pivot, had that been Fred & VDB for example the balance might have been a lot better as I think VDB would have naturally drifted further forward and we would have retained possession more successfully, which is how you avoid getting too tired in these games. Just my opinion but that's the way I saw it last night - they just pinged it around their midfield using the extra man advantage and we tired out 2nd half, they were lucky with the set piece but generally had complete control of the game and bar one great block from Diaz we didn't really do that much at all in 90mins..
I like both players too but they're obviously limited in what they can bring for certain situations