Can't say them not knowing them at all, or them being outright lousy.
When it come to CS fundamentals, algorithmics, most were better than me, who never studied CS academically. This was the reason we ever took interest in them.
It's just you cannot compensate for training, and experience with overall feel of skill. You can hire an ACM olympian, and the guy will still loose it unless he did C professionally for 3-4 years.
In my engineering degree doing stuff like B-Trees with its own i-node management module in C, validate against unit tests written by the professor, were requirements just to qualify for the data structures exam.
Someone lousy in low level coding won't go through such a curriculum,
Likewise those Webdevs might just brush up their skills for WebAssembly.
At the end of the day it doesn't matter if one is using D, Rust, Swift, Java, C#, Rust, C++, Zig, FreePascal, CUDA, Open... instead of C.
The system programming concepts are the same, and it all boils down to education.
As for lousy candidates, I get them in any language.