It was no slip.
Future Ted referred to Robin as "Aunt Robin" in the first episode. This is the twist ending of the pilot.
He later refers to Marshall, Lily, and Barney as "Uncle Marshall" Aunt Lily" and "Uncle Barney" innumerable times throughout the series.
I take it to mean (now) that these are terms of affection rather than indicators of any relationship by marriage to Ted's or the Mother's family. (Of course, it's easy to say that as we didn't know for sure through most of the series. With only a one-hour/two episode broadcast to go, I'm fairly confident about it now.)
Holy moly! The Slap Bet came back in true form, and it was useful! Way to go, Marshmellow!
Also: the final narration from Future Ted works really well, not only here in the space of this episode but also to us, the audience from the show. Just like the poker scene in "All Good Things..." on TNG, this was HIMYM's way of giving us a thank you for sticking with them all these years, before we go on one last adventure with the gang again:
Future Ted said:
Kids, I won't lie...that was a long weekend. More ups and downs than I can count. It was a twisting, turning road that led to the end of the aisle and not everything along the way was perfect. To be honest, not everything to follow would be perfect either...but what is? Here's the secret, kids...none of us can vow to be perfect. In the end, all we can do is promise to love each other with all that we've got...because love is the best thing we do. And on that lovely spring evening, that's exactly what Barney and Robin vowed to each other...and it was legendary.