Did the delivery work functionally as expected? If yes, then they understood what they need to understand to do that work on code. You don't need to understand the whole codebase, just the parts relevant to the work at hand.

I wrote complete systems that run businesses today. I understood every single line of code.

I'm not sure your experience is the norm.

I'm really glad that wasn't my career experience. I worked with a lot of extremely sharp devs in a variety of places. Dull ones were simply not hired.

Then blame the (worst) workmen, not the medium.

Ive worked and work with people who I dont consider super talented but sure as hell they understand the code, maybe just some lines, maybe the method, maybe the class, maybe the module and further.

Unless you worked at the shittiest company in the world, lets try to not bring others down just because they weren't as amazing as we are. They might have other skills weve lacked.

E.g. like a journalist complaining that they colleagues didn't publish books... who cares? Thats not their job, they need to write clear good concluded articles. If they write their articles like books, nobody reads that shit.