Well, right, that's obviously what happened, except that Cap probably didn't time travel forward after Peggy died... I think he just waited around for 7-8 years. That's why he was sitting on the bench instead of reappearing on the platform.
But I think Rob's question was about whether this wipes out Cap getting frozen in ice and everything he did in the MCU... I don't think it does - I think there were two of him, just like there were two Hulks when they went back to 2012, but thinking about time travel always makes my head hurt.
Yeah that's what he did. He was around 35-40 during Endgame, so going back to 1970 gave him another 55 ish years, so puts him around 90-95. The serum will make him live a little longer than most people so that's about right.
As for the 2nd Cap, yeah there are two. There's the one that thaws out, and then joins the Avengers which keeps the original timeline together, and then him, who is just Steve Rogers, and isn't doing any Cap stuff as he lives out his life with Peggy. He'd be like 65-70 when things start kicking off with the superhero stuff, so he'd be a little too old to be in the game anyway.
If he had gone back to WW2 and stopped himself from crashing the plane, then yeah it would have messed with the ice thing, but he's just a little tiny addition to the timeline, not a whole extra branch of one.