The only thing more frustrating than a rustacaen redefining "safety" to suit their purpose is a Stallman discipline redefining freedom to mean something totally weird. Something in the public domain is not free? Give me a break.
Copyright law is tricky. Under US law, "public domain" means "not copyrighted", the author waives all rights and the law allows them to release their works to the public domain. This you can consider free.
In other jurisdictions, say The Netherlands, the "Auteurswet" does not have a way for you to release your source code into the public domain, making any such statement void. Other countries such as Germany do not allow the author to waive all rights ("Urheberrecht"), etc etc.
Without knowing where the software was written, by whom the software was written and with whom it is shared, you cannot guarantee anything from "public domain" software.
Rust code without unsafe blocks guarantees memory safety, and when you're writing unsafe blocks you're culturally expected to expose a safe-api that upholds rust's memory safety guarantees for users who don't themselves use unsafe blocks.
Rust programmers (rustaceans, a pun on crustacean because the language mascot is a crustacean) tend to extol the virtues of memory safety [1], and are sometimes lax with using the word "safety" to refer to "memory safety". There are obviously non-memory-safety safety considerations as well (e.g. the other 1/3rds of security vulnerabilities. Or all the non-software uses of the word like making buildings that don't fall down).
Personally I think it's usually pretty clear how the word is being used, and don't find it annoying. But then I'm a rust programmer.
[1] E.g. pointing out that roughly 2/3rds of security vulnerabilities are caused by the lack of it.