FDB green thumbs, what's going on with this orange tree

It could be a freakishly large lemon...I think the variety is ponderosa that gets stupid big. But it can also be a citron, aka etrog. The citron has ritual use during the Jewish holiday of Sukkot. We're the previous owners of your house Jewish? Once it ripens, you can cut into it and find out. A lemon will be juicy, a citron will be dry on the inside.

Grapefruit are usually rounder with smoother skin.

Another possibility is that you have a young satsuma tree producing what is colloquially called puffy fruit. Young trees sometimes set fruit that doesn't fill with juicy pulp, instead it is mostly white pith under a crazy thick skin. When you feel on the fruit, does it feel soft or firm?

Judging by the thorns some of the tree might have reverted to root stock.

Also, usually thorns are a sign of meyer lemon/lemon tree. Although I wouldn't base an opinion on the presence of them.

I didn't but we just moved into this house. So the previous owner may have.

Again, it may be fine or that outgrowth may be coming from below the graft line.

Usually though, if the tree sprouted thorns that size it meant that the root stock had taken over and the fruit is likely to be compromised.

It will be fine to eat, but may be a lesser desired orange/lemon.

The orange I tried was a little tart..

Really? I tried one orange and it wasn't terrible

Tree nuts

my belief is you have the wild rootstock of a Satsuma growing and not the grafted tree. The wild Satsuma (root stock) is very thorny and I believe the fruit photo you show is the wild version.

I've seen plenty of rootstock growth, and yes, it is very thorny. But it rarely produces fruit, and the fruit is typically small/stunted and dry/bitter. It's easy enough to tell if this fruit is from the rootstock or the grafted "good" growth....follow the branch bearing fruit back to the trunk. The trunk will show a noticeable thickening where it was grafted--it will look sort of like a knuckle on your finger: a gnarled spot. Does the odd-fruiting branch emerge from above or below this graft? If below, it's the fruit of the rootstock.

RE: rootstock taking over, you don't need to uproot the whole tree. Just cut back all the branches that are growing from below the graft. Remaining branches will still bear good fruit.