Is "resubstantial" even a "real" word?

I'd say it is not a word in common usage, but could be a constructed word to suit a specific purpose. I think there are times when you are describing something that has no word for it, and you can construct one by adding prefixes or suffixes. When we do that, we are introducing a new word to the reader and we tend to add a hyphen to indicate that it is a construction of the writer's device. If that word enters common parlance, the hyphen will eventually begin to drop and then drop altogether. Examples might include to-day, tele-phone. Don't quote me, I am just trying to recall some examples.

So if that is what is happening here with re-substantial, it would be a new word that builds on the prefix re- meaning to repeat or to be again, and the base word substantial meaning of substance. This combination could be applied either in the physical world or in the realm of ideas, since substantial is applied both ways. Physically, a substance could become less substantial and then become fully substantial again, or re-substantial. An idea could fall into disuse and then get picked up again, becoming re-substantial.

Just guessing here.