Reason why this is said about Casemiro especially isn't that Casemiro is actually slow for a DM. It's because of his relatively weak positional sense and situational awareness mainly because he wants to be someone who contributes with goals and assists as well . I'll use Yoro (I know not a DM) as an example of how to defend without always being on your behind and without always having to chase play because he focuses on where the danger is going to be before it happens where as Casemiro tend to always need to catch up hence needing to be a lot more mobile than he is to make it work.
A guy who sits at the base of midfield, is fairly strong with decent passing ability and even the level of Casemiro mobility would be fine if he had the situational awareness and positional sense.
Yes recovery pace is a good weapon to have but Bosquet, Carrick and many others didn't have it but they were smart positioning.