The trick with the steam engine (or any other old tech that needs to be re-invented), so that we all know what it IS, but can you build one?
For the most part, we've moved past the industrious, handy people of the past, and we're just consumers at this point. Most of us don't know how ANYTHING works, not really.
Sure, you could name plenty of people that could figure it out, but when you kill 90+ % of the population, how many of them were the tough, scrappy, aggressive types that would have survived the mass die-off? Not many. And you'd need a setup to build these kinds of things, and again, good luck.
They've talked about how rare bullets are, but I'm not convinced that muskets would be any MORE common (much less, actually), and then you still need supplies to make it work, a cache of gunpowder, etc.