I am sorry to hear of this, a loss of this nature is a loss for the entire community. We tend to blame the equipment first, but usually, it's only one link in the accident chain.
His foot may have slipped off the clutch but there are a half dozen other remedies that can be rapidly done to deal with the situation. If you are running so close to the margin of safety that you don't have time to react with a backup or a slip of the clutch foot is dangerous, then you are operating the tractor unsafely.
The HST is a great invention and offers some added safety in certain areas but takes it away in others. I've said this before, just as an example of danger here, when you can easily go forward, reverse, add or remove speed, it can make you complacent and complacency is far more dangerous than a foot slipping off of a clutch. I'm not saying you will get complacent, only that it's a greater risk. A gear tranny forces you to be honest with your workload, your health, and your experience. That doesn't make it worse or better, just what it is.