... okay, having had a close family friend that was an actual small time gunsmith (until alzheimers got 'em, anyway; they're still alive, but yeah.), no, gunsmithing and whatnot (assembly, modification, etc.) is not some kind of impossible task. Hell, maintenance is usually pretty simple and only rarely really takes tools beyond what you might have around the house normally.
Doing anything precise with gunsmithing (y'know, that part that makes guns even remotely reliable), however (like, hey, doing anything even remotely resembling a substantial modification), is something that's significantly more difficult than a whole hell of a lot of things, and something that takes both a great deal of effort and no small skill. Extremely difficult may not be the word for it, but it's not some task that's even remotely easy, either.
Don't underestimate how difficult it is to have a gun that's not complete shit without having both pretty good tools (not specialized, iirc, but it's not unlikely your average small shop just isn't going to have tools that are enough up to spec to let you do serious modification and still reliably prevent things like, y'know, the gun exploding) and a damn good idea of what you're doing, is what I'm saying.
If the gun's built to be easily modified it's one thing, but if it's not, stuff just got complicated. Not impossible, just like making an IED in your basement is not impossible, but complicated in a number of different ways.
E: All that said, do we actually have any news about the AR-15 being modified, yet? ATF's twitter's last statement is still just that it was legal.
E2: Ah, it apparently wasn't an AR-15 at all. Look downthread a little for a link.