The power Psykos obtained was most likely due to absorbing some of Saitama's power, who is still in the basement. Tatsumaki herself wondered where this huge power comes from and that she couldn't possibly obtain such power by simply absorbing monsters. While asking this, we saw the moon in the background with the crater done by Saitama jumping off of it, which implies this as well.
Psykos herself also asked, whether "that guy" that appeared during the fusion was real.
Lol looking at the translation, the chapter sounds completely differently and makes more sense
> It's implied Psykorochi became that strong due to absorbing part of Saitama's power
> Psykorochi is portrayed out of Tatsumaki's league and it seems she was aware of it
So based on that then, this fusion is probably just a temporarily boost. I'm assuming it will get stopped by something / someone, either due to Psykos failing to control Orochi or it getting interrupted by outside power cancelling the fusion
It also shows that reading the raw and reading a chapter in English can change one's perspective completely