Even the simple Schwarzschild solution for a non-rotating and non-charged black hole has a region of "asymptotically flat spacetime" on the "other side" of the black hole that looks like another universe. It doesn't seem like you could get there though.
You can find paths that lead from our universe to those other universes in a rotating black hole but it is not so clear you can survive the trip. The inner event horizon and the "ring singularity" inside the Kerr solution are believed to be unstable classically and thus not really there. (Personally I think the classical black hole is probably entirely wrong on the other side of the event horizon.)
Andrei Linde wrote a paper on "chaotic inflation" that suggested the inflation process is driven by topological defects that have a very high energy density such as magnetic monopoles, cosmic strings, and domain walls. Inflation is ignited by the presence of such things and the result is that you don't see any in our universe, if you are lucky you might find some evidence that one of those is near our universe that is imprinted in the cosmic microwave background.
One could imagine that in the violent processes inside a black hole that inflation would activate and create the space for a new universe or universes. I'd imagine you'd have to go through that inflating region to get there and I think that would be both impossible and deadly.
That joins many insinuations that there are surfaces around black holes (e.g. "firewalls") that will kill you if the spaghettification doesn't.
If you think black holes spawn new universes you might imagine our own universe was "naturally selected" to produce lots of big start that make black holes, which is an interesting twist on anthropic arguments.
Wouldn’t this imply a shrinking/inflating equivalence principle? Matter inside parent black hole in parent universe 1 is shrinking from perspective of universe 1, but space is expanding from perspective of inside parent black hole in child universe 2.
Are black holes write-only or is any information fed into them destroyed? And by information I mean even like “let’s do binary by shoving ten thousand stars into one to represent a 1 bit”
Seems like the consensus is that the information does get out by the time the black hole evaporates, and probably via the Hawking radiation (there's not really another mechanism), but exactly how is contested and may be untestable.
By how much could the event horizon of a black hole with the mass of the universe deviate from the size of the universe before the similarity is no longer remarkable?
The author of this article doesn't seem very keen on citations.
I'd also like to know a related answer: are these two diameters the same because they're derived from the same model? Isn't the mass in the universe estimated from the observed diameter? Then take that mass and calculate the diameter of the hypothetical black hole with that mass, if the physics is the same, would that result back at the observed diameter of the universe? Or are these two completely separate models that just so happen to be similar and genuinely point to an insight?
But, is X? Well, we don't know, therefore we don't know Z. It's fun to think about, though!
The headline question is, predicably enough, never answered. The whole thing is just a long restatement of the question, followed by "dunno, lol".