In D&D 5e, is the inside of a bag of holding actually described anywhere as being an extradimensional space/demiplane?
The description of the bag of holding doesn't actually explicitly state that it is, it only implies it with its mentioning of "This bag has an interior space considerably larger than its outside dimensions", and talking about scattering contents into the Astral Plane and what happens when you combine it with certain other magic items...
Speaking of which, the handy haversack says:
This backpack has a central pouch and two side pouches, each of which is an extradimensional space.
and the portable hole says:
You can use an action to unfold a portable hole and place it on or against a solid surface, whereupon the portable hole creates an extradimensional hole 10 feet deep.
The cylindrical space within the hole exists on a different plane, so it can't be used to create open passages.
Folding the cloth closes the hole, and any creatures or objects within remain in the extradimensional space.
In other words, these other two items explicitly state that they create extradimensional spaces, and the portable hole even goes as far as stating that the space is another plane. The bag of holding description says no such thing, leaving us to simply infer that it probably is an extradimensional space.
Am I missing something? Is there something more conclusive rather than my above inference that the inside of a bag of holding is actually an extradimensional space?
I ask because this came up in our last session; a player and the DM discussed it, the player arguing that it was, and the DM pointed out that the DMG didn't actually say it was, although the DM eventually decided to go with it (especially since a couple other players, myself included, piped up with that we had always been under the impression that it was).
The player arguing that it was has played previous editions of D&D, so maybe it was explicitly stated there, which I would be interested to know, but I'm primarily after any 5e evidence that it is besides the inference included in the question.