Yeah... it's pretty much unquestionable that Myron invented Jet. He goes into a pretty lengthy description of it's history, development, how he was hired to make it, the discoveries that led him to Jet - pretty much everything you'd ever want to know about Jet. You can even get him to work with you in coming up with an cure for Jet addiction (as the original draw to the drug was that it was the one drug in the Wastes that you couldn't ever kick.) I mean... he's Myron baby!
I don't think it's all that unrealistic that someone could have reverse-engineered Jet, and that there's some mysterious factory outside of DC that's pumping it out by the truckload. My only problem is that Bethesda, for all their touted interest in building games with lots of lore, didn't see fit to provide any mention of how it got there.
What's the Brotherhood doing in DC? They explain that.
Why is the Enclave there? Explained as well (maybe not as to how they're still around - again, another rationalization with no attempt on the behalf of Bethesda to supply the background, but still... there's at least some attempt.)
Want to know why Harold is in DC? You can find that out.
Why if there FEV in Vault 87? (Well, apparently there's a terminal that explains it all, only you can't get to it in the game. But okay, they at least get partial credit, and maybe that was just an oversight - they're only human, after all.)
But Jet? No, you just have to take their word for it. It's not so much that it's in the game; as that if they decided it just
had to be there - then why not go the extra step and put some background info on it? It kind of feels like that "missing reel" segment from Robert Rodriquez's Planet Terror, only you know, not intentional...
"Oh no no!" says Todd Howard, "we
have to put Jet in the game. It's just got to be in there, no two ways about it. Oh, what's it doing there? Oh no, we can't explain that, Jet is where I draw the line, buddy!"