I'm quite acquainted with said knowledge. A small community surviving with a garage full of machine tools, some glassblowing, and a bookshelf of dusty encyclopedias, could easily bootstrap back to the early 20th century. A bit of steam reforming and the gigatons of plastic trash gives an ample source of chemical precursors to replace petroleum for chemical uses, and biofuel/gasification for energy. Not to mention millions of solar panels for the taking.
Just because it's not economical to do these things now, does not mean it would be impossible, especially in dire straits.
Would it cut the human population to a few million? Probably. Reset to the middle ages? Hardly.
What are the real innovations that make the modern world?
- the surface plate and 3-plate method
- steel metallurgy
- certain key chemical processes: Haber, Bosch, etc
- electrical generation
- the radio/telegraph
- the transistor
- calculus and algebra
All would survive a full-blown nuclear attack. Rebuild would be slow and tedious, but completely plausible, if not guaranteed.