RT, it sounds like a Catch-22 in that you won't know what it's like unless you try it (for bleeding obvious Opsec reasons) and it's only by trying it that you'll know if it's for you or not. Have you considered arranging another interview with them, and asking them what you've asked here? You still might not get much of an answer.
The Int Corps Tech selection will give you a good idea. If you're a qualified soldier less than 3 years out then you won't have to do the generic TA Selection. A weekend of your life will give you roughly what you need to know on whether to take it any further.
I left when it was Int & Sy Gp (V) so I'm well out of date but if it were all 'paper sifting' the guys would loose interest and walk; they didn't then and I'm sure they don't now. Try it and see and, as has been said, if it isn't for you you can leave.
MI is a broad church - if you want analytical, critical thinking-type opportunities, they are there. Contrarywise, there is stuff for those who want something more strenuous.
I started my TA career with Int & Sy Gp (V) in '90 then went onwards and upwards - well, sideways more like, hence the avatar ! One of the best things I have ever done and as a bonus it has given me skills that I use in the day job.
I'm in and I'd echo the advice here - get on touch, ask questions, do the selection weekend and see if it suits. The vagueness isn't an affectation, it's a reflection of our very close links to the regs and real work and the huge range of possible things you could end up doing.
Trust me, it's great. No, honest. Just don't ask me about being a LCpl unless you want some boring rant about 3 Shock Army, wooly pullies and the smell of damp '58 pattern webbing.