You missed this part
"Think back to the best midfields over our lifetime (
not as rigid as I make it seem, just bear with me). The 4 things you want accomplished are passing, defending, attacking, and providing energy."
As there are 4 things needing to be covered, players can cover a certain part collectively. In the case you highlighted, Vidal provides a lot (I mean a lot) of energy to compensate for Pogba and Pirlo, which allows him to contribute defensively. You combine this energy with Pirlo, who although lacks energy, stays back to compensate meaning he can utilize positioning to help out defensively. This is similar to Matic's role except Pirlo is a better passer, same with Carrick in his later years. Combined as a trio, defense isn't left wanting.
I specifically mentioned different types of combinations because there are multiple ways of accomplishing the 4 tasks with only 3 players forming the midfield. Look at Kroos and Modric, I listed two passers, because unlike Pirlo, they're more mobile. They can go forward to help out whenever needed, and they don't technically need a Vidal type who has bundles of energy.
Our midfield literally lacks 2 important things to make up a class midfield.
*Another edit
Just compare our midfield of Pogba, Herrera, and Matic compared to Pogba, Vidal, and Pirlo. Herrera was doing Vidal's role, and Matic was doing Pirlo's role. As Matic and Pirlo aren't defensive specialists, they also need something else in their locker. Matic has nothing so if he's not some defensive specialist, you can upgrade him. Pirlo was a brilliant passer. Although Herrera had Vidal's energy, just look at the attacking output he had compared to Vidal. 15 and 18 goals in 2 of his seasons with Juve.
Leicester are accomplishing this except not to the level of those all time great sides. They have Maddison, Tielemans, and Ndidi. For as decent as Scott has been, what exactly does he excel at? Thankfully, when our midfield is as bad as it is, what Scott provides is necessary right now. However, if we upgrade, his importance will dwindle unless he improves in attack, passing, or defending. Fred? He's trying to pass through the lines and does have energy, but isn't some specialist in anything. It's clear how much we lack in quality for 2 of our deeper midfielders and with Pogba injured, in all 3 midfield positions.