The next sentence, he elaborates what he means. He goes on to talk about "midfielders who are complete". In other words, total footballers rather than specialists.
Nothing wrong with that.
What this basically means is that he doesn't believe in the likes of Makalele having one specific job in a 3, or Lampard having one specific job in a 3. Instead, he wants all-rounders. Hence the pursuits of Bellingham last season and Camavinga this season. Those two are the future of football. Anything but throwbacks.
Solskjaer's philosophy is spot on, imo. The more limited the individuals are, the more exploitable their weaknesses become. The ideal midfield is one where everybody can do a bit of everything and is a constant threat to the opposition.