The main idea is that - Zinchenko is the type of fullback that team needs right now..
Since Fabian Delph as a pioneer, Pep I think has found out that - you can have quality fullbacks like Mendy or Walker for example but that's not unique in this league and world. Because there are Lucas Dignes, Arnolds, Robertsons, Davises and similar players - who all do that standard fullback jobs: play wide, make crosses, run and etc.
But with Delph, Cancelo, Zinchenko - Pep has / had something unique and that's what makes the difference.