How is a "guarantee" not claiming something is 100% ?
"I guarantee I will be there" - but I could always be hit by a bus, or have a very serious family issue to tend to, or an earthquake might happen, or the airports might be closed due to Covid and so on.
"Our bank guarantees your money" - yeah, except if the global economy collapses, or the country is hit by an asteroid, or if there's martial law, and so on.
The trivial such cases are irrelevant to the guarantees they want to offer (and same for Rust), and it's a bad move to point to them and consider them as part of his argument.
Not to mention they're saying "trying to", not guaranteeing in the first place. Which acknowledges things like possible bugs, or some edge case not handled, etc.