My understanding is that anything inside an event horizon is swiftly collapsing inwards towards a central singularity (or at least that’s as much as the math can tell us). Since this does not match the observed behavior of our universe, why is it considered a valid hypothesis that our universe is the interior of a black hole?
As I understand it, this is Black Hole Cosmology. The idea is that there’s an 11-dimensional overall universe, in which black holes form collapsing to 9 dimensional universes, in which black holes collapse to 7 dimensions, and so on, losing two dimensions at each generation. Our black holes have only 1 dimension inside.
This goes on until somebody else here who knows better comes along to straighten it all out.
It doesn’t match our current observed behavior of the Universe, so nowadays, if you want a black hole cosmology, you need a more complicated sort of black hole (in particular, black holes are much weirder when the cosmological constant/dark energy is relevant). It was much more popular a few decades ago, when we did not yet know whether the Universe would eventually collapse into a Big Crunch. In a simple black hole cosmology, the singularity at the center of the hole is the Big Crunch.
I think this somewhat relates to an older idea (from several decades ago). The idea was that perhaps the total mass-energy of the universe might equal the total gravitational potential energy. So in a sense the universe “came from nothing”.
But I think this was before observations led to the postulation of dark matter and dark energy… at the time baryonic matter was the only thing known.
So probably not taken seriously nowadays?
I was expecting to read something about holographic cosmology, how the universe’s physics are encoded as information on the boundary of the interior of black hole and that thus our universe is actually a hologram … and hoping that the explanation would be understandable to me this time!
No I guess.
Who exactly is considering this to be a valid hypothesis?
Gift link.
But as said, I am not really understanding it. 
The black hole one, the holographic one, or the combination of both? Both of them are not so much hypotheses themselves, as collections of hypotheses. In both cases, the simplest version of the hypothesis has now been ruled out: The black hole hypothesis by dark energy, and the holographic hypothesis by the noise (or lack thereof) in one of the gravitational wave detectors. But in both cases, there are more complicated versions that are still not ruled out.
I’m not qualified to either support or refute black hole cosmology, but I’ll just say that, simplistically, there are no really obvious ways that it doesn’t match the observed behaviour of the universe. Because the behaviour inside a black hole is likely very different from what we intuitively might imagine.
The Einstein field equations that describe the geometry of spacetime in the presence of gravity lead to interesting results as one approaches a black hole. As one approaches the event horizon the curvature of space becomes enormous and time slows down dramatically relative to an outside observer. At the event horizon, in theory time stops completely.
What happens when you cross the event horizon is unknown, but if extrapolations of the field equations are valid, it’s plausible that time and space switch places, and the radial dimension towards the singularity is now timelike. You cannot escape from the singularity, not so much because of its gravitational attraction, but because you’re bound to the arrow of time.
If one applies this concept to a hypothetical black hole universe, we see that only one of four dimensions of spacetime has an inexorable fixed direction that we can’t reverse, and that dimension is time. So why isn’t everything in the universe obviously falling into the singularity? Maybe it is, because it’s all bound to the arrow of time, just as may well be happening inside black holes.
I’m not saying this supports black hole cosmology, or even that it’s scientifically plausible (I welcome any criticism), but only that the concept of the universe as a black hole is, at least superficially, not as crazy as it might seem.