I think he was debating whether what’s currently unexplained can be called unexplainable. That assumes we have all the possible knowledge in that area, if the word is taken literally (though I’m not sure that was your intention).
If I see something and can't explain it, I may describe it as unexplainable. It could possibly, actually be unexplainable (the actual word is ineffable). But it is far more likely that it is explainable... just not by me.
It's an interesting topic, because central to the concept of orthodox christianity is the idea that God is ineffable. But that would then call into question how one could describe God. If we can describe a being that is indescribable... than by definition, it is not indescribable. What's really cool is that this same logic was used in reverse in a very old (like 11th century) ontological argument for God that is basically:
God is that than which no greater can be conceived.
If you can conceive of a being greater than God, it isn't God.
If God doesn't exist, you can conceive of a being greater than that being.
A being that exists is greater than that same being which doesn't exist.
Therefore, because God is that than which no greater can be conceived, God must exist.
It's been a number of decades since I've thought about this one, but that's basically how I remember it. Now, there are a lot of obvious issues with this argument. But it's stuck around in various forms for a long time, so, it's got some legs. But if God is ineffable, and greater than that which can be conceived, how does one worship that God? How can one believe the interpretations of that God's will in churches? Etc, etc.
To the main point of this discussion is that this argument of ineffability would be unique to our experience in the universe, in that we see evidence every day that things which are as yet unexplained are not ineffable, but rather are just not known to us yet.
