Alicia is right that this is not a black and white milestone. Construction work will continue (heck, there's multiple active construction projects going in a typical theme park that's been open for decades). I believe it's likely more of a change of "who's in charge of the site" in terms of priority, scheduling, traffic control. So, construction teams will have to get clearance for their activities from Operations, with Operations calling the shots as to what is allowed, when. This makes sense because when you talk about guests starting to be onsite (even TMs), they have to take priority...especially their safety. Operations will be able manage that better than construction.
But it is a key milestone, and it's very exciting to know we are close to hearing reactions and learning more what's going on inside all the buildings!