With degrees around like Media, Football, and David Beckham Studies at one end of the spectrum, and Chemistry/Engineering/Mathematics etc at the other, I would have put ATPL theory at somewhere in the middle, difficulty-wise.
However, I agree that the timescale of ATPL theory is much less than a 3-year degree - if you're in full time-study, ala Uni, I'm sure you'd not need more than 1 academic year to do it in.
(speaking as someone who has experience of Aerospace Engineering at Soton Uni, has just done all the ATPL exams part-time, and is off to Kingston's 3rd year degree top-up/ATPL course this year).