I think regardless of the school and instructor's difficulty, there's a group of people who basically make perfect grades in STEM topics (or all topics) regardless of those variables. And for some people it's easy to know what their grades were 20-30 years ago because their experience was "I made effectively perfect grades at everything my entire school career" and will vividly remember the couple exceptions. I didn't pride myself on my grades and it wasn't encouraged at all by my parents or peer group, but I was one of those students, and have the experience of remembering exactly the one class I got a B in (photo journalism, the teacher hated me). I don't say this to brag, this account isn't tied to my name, but more just explaining what Canonical is perhaps looking for.

I do think there is a large connection between grade performance/SAT scores and general aptitude and success later in life. But as you said there are plenty of circumstances that would lead to poor grades that aren't related to ability - but I would also say those circumstances are also possible indicators of how good of an employee you will be. For example having behavioral problems as a kid could lead to bad grades despite otherwise capable of good grades, and I would suspect there's connection between childhood behavioral problems and conduct as an adult.