When you remove the context around statements (thread about rust concurrency), it's easy to make a statement into overblown nonsense. You may be interested in the broader analysis contained in the link from parent comment, about the type of safety being discussed in this thread, or you may not.
The “unsafe” keyword in Rust is a source of perpetual oblivious overconfidence. It should’ve been called something that befits it’s actual narrow scope (`disableStongTypeChecks` or some such) but that ship has sailed.
"unsafe" is pretty clear: the block/function is not safe, that doesn't mean the rest is safe. I suppose it might have been better named as "asserted", "assumed_safe", or "manually_checked", but that's much more verbose and not necessarily clearer.
These blocks are supposed to be rare, so a more verbose keyword isn't a problem. It would be better to be precise here and avoid a concept so general as safety.
This statement does not make any sense. That, or we have vastly different definitions of what “safe” means.