While I'm sure others here will have a better answer, in my 14 years with Hilton affiliated properties, early check in has definitely never been a guaranteed benefit and that I'm aware of, neither has late checkout. The internal verbiage has always been that a Diamond member should get preference for late checkout based upon availability and I have heard from members about how "at one time" it was different, and maybe it was, but in my time, I haven't seen or heard something to the contrary.

Fwiw, at the HIS I'm at now, we're fairly tight with late checkout especially on high turnover days like Friday and Sunday. We'll do maybe three max and never past 1pm. But, that changes daily based on who is working the desk and how many housekeepers called off that day. Plenty of days where the answer is zero for late checkouts.