The risk-of-death is actually quite well understood. If a user gets too much insulin, and doesn't feel when their blood sugar is low and doesn't get a low-blood-sugar alarm or doesn't respond to the alarm, then they could die. So if the closed-loop system incorrectly commands too high a dose of insulin, that could be fatal.

But the alternative isn't that too-high doses of insulin never happen. The alternative is that all insulin doses are selected by a human, and subject to human error. And having a low blood sugar event due to too much insulin is not a rare occurrence for T1 diabetics at all.