We can't know that yet, universes if parallel may interact at small scales in such a way that they can't be detected and reachable. To say they're unreachable by definition is hubris.
edit: Also Xcelerate has confirmed thats what they meant: https://news.ycombinator.com/item?id=12083025
> If there is another universe out there, it is by definition unreachable from our current universe
As this quote by definition accepts the distinction of multiple universes as it makes zero sense otherwise. To take him at his word means he believes it's possible that there is another universe, but that it cannot be reachable from our own; no where does this statement imply he believes there's only one possible universe and it directly implies otherwise.
His correction merely makes it clear he can't express himself clearly because what I just quoted above does not and can not make sense from the position of there being only one universe.
I'll even say props to you for parsing that nonsense because what he said in no way matches what he claims to believe.
Wow, harsh. Sorry to have aggrieved you so much.