IIRC a key obstacle why steam engines were not used earlier despite the concept being known for at least a millenium was the requirement for quite advanced metallurgy - you can make a nifty proof of concept from copper or iron, but a useful steam engine needs to be (a) relatively high pressure and (b) large, so you can do it only if you can reliably and cheaply make large quantities of decent steel. If you can't make large quantities of steel, your steam engine doesn't work; if your steel-making process has unpredictable results, then your boiler blows up at a weak spot, and if that steel is expensive, you're better off having the same people work a literal treadmill instead of making a steam machine.

At least with iron, you'd have the benefit of the existing refined ore lying all around you in a post-apocalyptic setting. There's little need for actually mining iron ore anymore if your population has been reduced by 99% or more. You can walk down any abandoned street and find sources of iron and other metals. Now, there's still the refining process (but it would be shorter from something already processed) and fuel to contend with.

Also, making glass is not just combining sand and seashells and fire.

I don't doubt that they could have made glass plates or something, but they start turning out vacuum tubes and borosilicate beakers next to each other like it was all a matter of knowing the recipe.