The double pivot doesn't require a pure DM. We should target players who are adept at both defending and attacking in the double pivot IMO. So I do agree with what you're saying. And say we had a double pivot of Bissouma and Camavinga, that would allow us to transition play effectively from a deeper role compared to what we're seeing currently, and also give us flexibility as far as changing the formation goes, from a 2 man midfield to a 3 man midfield.
I also don't think Pogba suits playing in the double pivot, hence we see Fred and McTominay playing as a pair. And that's mainly due to our CBs not being strong in 1v1 duels higher up the pitch. So when people say our defence is fine, then also credit the McFred pairing who are baby-sitting the CBs.
So if Solskjaer wants to successfully implement his high tempo, high press, fast transition style, he needs to upgrade at CB, to strengthen the foundation of the team, which would create the conditions for him to implement his style of play. So that would free up the midfield to help cause overloads and provide the counter pressing option. So it makes sense to me as to why Solskjaer would prioritize the signing of a CB, if reports are to be believed from the likes of Ducker, Stone, Cooper etc.