Thought (aka totally inaccurate fan theory):
That wasn't Steve at the end of Endgame. Steve doesn't age. In fact, Steve is very much alive. He delivered all the stones back to their original places. He also brought Nat's body home for Yelena, so that's not a cenotaph. And he lived out the rest of Peggy's life with her after he accomplished his task.
But Cap doesn't age. At least not like a normal person. So that's not Steve talking to Sam.
So, my explanation? The theory about Cap being on the moon in FatWS? True. Maybe he's working with Fury and Carol.
So who was that guy who handed the shield to Sam? Possibly someone who had posed as Steve before. Someone who is supposedly dead, and is supposed to be living on a barren, lifeless rock in the middle of nowhere out of contact with everyone, which actually sounds a lot like Vormir. Someone who got bored and when the overly patriotic, annoying human showed up, decided anything was better than being stuck there any longer.
Just a thought. ;)