Mentzer's HD2 template basically had quirky frequency setups in his routine. For legs and back, he would have you work out once every 12 days. But the upper torso would be worked twice over a 8 day period, and then given a 8 day layoff.
What this usually meant was that both the back and legs were significantly deconditioned session to session. This, in addition to working with new PRs for legs and back, would guarantee significant microtrauma during each session. And because we're talking about a very low volume one-shot, it's not likely the muscle's resistance to future bouts would have been adequately developed anyway. Thus, each growth response would have been very significant and possibly sustained longer than the 36-48 hour window.
It should also be noted that cardio was strongly frowned upon with his program. The HIT conventional wisdom at the time was that cardio ate up gains, not only glycogen storage. Here, this was somewhat true. Due to HD (and 3-way split routines were designed in general) very, very strong reliance upon a significantly deconditioned state to elicit growth, any amount of serious physical activity would significantly hurt potential gains. Looking back, it's probably the rise of the MWF split in the 90s that begun really demonizing cardio as this mass-eater. :o
The other thing is, general arm and upper torso gains, strength or sizewise, were not that great with HD2. People usually saw great gains in the back and leg work, but the arms were the first to taper a bit. In fact, I think the static contractions were introduced mostly to bump up arm development. (Though to his credit, Mentzer recommending hitching the stretch reflex in his movements too.) And again, the conventional wisdom was that, well, arms are a smaller bodypart and need more time to grow. And some people later on reasoned that arms didn't grow that much because the frequency was higher-than-optimal in his given template. People noticed that it was harder to continuously increase their bench press on HD2 whereas the legs and backs flew right by.
The problem was two-fold in his arm exercises. He chose peak contraction-style exercises, movements like lateral raises and tricep pushdown, which can induce a lot of stress (thus slowing down strength gains), but is often no better if not inferior to the bench press and other movements in producing microtruama.