It is related to Slacks' video
More to the point, the sister earth that was mentioned is Heroes of Newerth HoN, and voice lines of feeling regret to what happened to HoN is maybe, just maybe what Icefrog feels about HoN, like:
And with HoN's developers announced that the game will be shut down on June 20, 2022. We can expect / hope that Valve already made request some of HoN's heroes to be ported to DOTA 2 because Icefrog "allegedly" worked for S2 Games as a programmer on Heroes of Newerth, and maybe he want some of his creations back to him.
And from what I have seen from Valve's great work on the event games in last two years, I believe personally that the next update will be huge, and there will be rework to other heroes beside Techies like Meepo.
Source: https://www.reddit.com/r/DotA2/comments/swkm3c/theory_about_731_and_planetfall_arcana/