I believe it goes something like, "I have constructed a strawman that Rust claims that all code written in it is automatically safe by all conceivable definitions of safe, but look, ha ha, here's something that detects unsafe code in Rust!", and I don't mean "code marked in unsafe blocks".

It's a concatenation of several logical fallacies in a row; equivocation, straw manning, binary thinking about safety, several others. It's hard to pick the main one, but I'd go with the dominant problem being a serious case of binary thinking about what "safety" is. Of course, if the commentor is using anything other than Idris for all their programming, they're probably not actually acting on their own accusations.