I think the reason it states not to use a trunk bag is that its advertised as "narrow for better balance and control" and might be too skinny to support a loaded trunk bag in a stable position. I could be wrong, but that's what I gleaned from my reading about it.

I need to get out and do some measuring, because I've wanted to put a rack on my road bike for a while (I bought it used and it came with a rack, but the rack is FAR too close to me for it to be functional with anything other than a trunk bag). This might just be what I need, and it won't break the bank.