I think this varies depending on the person, but.....My own personal experience over the past 6 weeks has been cardio definitely cuts into my gains.
I used to run 4 times per week, 30 minutes per session, as I was always concerned with keeping fat under control. When I started HST, I cut back to 3 x per week on the running, only on the off days, and was feeling very tired most of the time. Well, long story short, I now keep my cardio at 3 days per week, but do ONLY incline walking on a treadmill for 20 minutes a shot. I keep my heart rate in the "zone" for my age, but find that my legs, and just about everything else, are doing far better without the running.
I've also noticed NO extra fat gain with the cut back, and frankly, am thinking of dropping it altogether. I'm starting to believe that cardio is not all it's cracked up to be, and does nothing but inhibit strength and size gains. (At least for me). My wife has been a prime example for myself as well. She cut cardio altogether 2 months ago and has gained more mass, and dropped more body fat, than when she was running 5 miles a day 6 days a week.
But again, in the end, I think it really depends on the person, and how your body reacts to different types of stimulus.