How did they get into college? I thought this stuff was pretty basic in high school. Please don't tell me a random ATOT dude who loathes college is smarter than half an advanced statistics class.
The slope intercept formula is basic high school stuff. But that doesn't mean you remember it.
You can absolutely get into (and out of) college without a math test. If your SAT/ACT math score is good enough sometimes they'll waive the math requirement altogether, meaning there are a lot of college grads in with degrees in non-STEM fields that haven't had a math class since high school.
I understand that the position of the majority of ATOT is that these people are not real college graduates and probably shouldn't exist.
Even if you didn't manage to waive it, you can take college algebra for non-majors 101 your freshman year and "D is for Diploma." Which is probably worse than somebody that managed a >30 on the ACT in the first place.
After, as OP mentions, 4 semesters of other math, they're at least Juniors, which means the slope intercept formula is something they possibly haven't thought about in 3-4 years. Possibly longer (assuming they learned it right in the first place.) It's entirely possible their particular professors didn't make much use of graphing, or relied very heavily on graphing calculators or Mathematica. If you emphasize "getting the right answer" over technique and process, you're fucked by the meta-stuff. (Personally, I think math should be taught without calculators. But that's old-fashioned me.)