Christ, it's pedantic central on this forum today.
I get what you are saying...if all the players are back, of course there will be more opportunities for a new manager to learn about the group dynamics. Similarly, what other people are saying is having a manager in a little bit sooner than a week before pre-season training is due to start would probably be helpful, to help them get acclimated to their environment. There will be some players around. Their will be youth players around. There will be staff to meet and understand how best to work with.
I can't even recall how this particular line of debate started, but it's another thing that has the whiff of 'things are absolutely fine!' when it's ok to admit that we might be facing a bit of a challenging time getting someone in that we really want. I mean...we are now literally arguing about whether it is useful or not for a new manager to have a little bit of time to get to know the club, rather than being expected to perform at their absolute peak with no adjustment period, from day 1. Why is this is a serious discussion?