Yes, it seems it is. As a biochemist student exploring biophysics options, it seems that for me I have to take E&M courses, and sometimes mechanics, before I get to take a course on electrodynamics or optics. I think these courses can be useful for me, but I suspect I might never use 90% of what I will learn in my professional career, and there's so many other courses that I can take that have direct applications.
For math and physics, programs are going to be very rigid. If I end finding the need for optics myself, I'd take E&M first to get a foundation. Even if you know the math, if you don't know the physics, I assume it is just going to be a hassle and probably a waste of time.
I guess you also aren't a physics student so maybe you are in a similar position. And in the end 'absolute prerequisite' means if they will allow you without it or not. I mean, if they allow student without E&M to take the course, but they fail 100% of the time, I guess it isn't an 'absolute prerequisite'.