I did not study CompSci but microengineering (mix of things, including robotics and manufacturing) and I am now working there as a SRE. Some colleagues are mathematicians. So definitely not only CS background :)

So in interviews do they also test mathematicians and engineers on undergraduate courses?

Yes. It doesn't matter what field you were in, it matters how well you know the skills they are looking for.

Had exactly the same interviews as my colleagues from a CS background.

It was frontend. I did ok with some heavy studying, but I'm definitely more UI focused on a day to day basis. The problems I had to figure out during the interview were nothing at all like what I do daily.

I walked away feeling like if I really wanted to I could knuckle down and study for longer and get a job there, but I decided it wasn't really my thing and went in other directions. Like I said, I took the studying and the subject matter as a challenge and I feel pretty pleased with how far I got considering I have a painting degree.