The notice that hast been posted says that if you have different arrival times and want to all arrive together, you need to show up at the latest time. The guard at the gate who checks IDs will also have your arrival time noted. If that time has not come yet, you don't get through the gates and will have to wait in a designated area. There will also be an area for those who have not signed up for a time.
I would imagine that, like the boarding cards, if you have an earlier time then you will be allowed in. Also, I thought I read that the busses unload at a different place and that if you had purchased
DCL transfers you don't select an arrival time.
There are many who have posted they are going to arrive early anyway or not sign up for a time. If someone intentionally does this to try to get away with it and "force" DCL to let them in early, it's not DCL's fault if their kids have to stand outside for two hours. The way it's been for many Dream cruises, people have been standing for that long in the sun and the rain anyway. This is DCL's attempt at eliminating that. IMO, it's not fair to the people who adhere to the rules to have to have their wait time extended because of those people who think that the rules do not apply to them for whatever reason.