Not really. It probably got overloaded with requests and thought that another crawler was tergeting the repository so it probably bumped up the measures.
Well github is very much owned by Microsoft, and with the recent changes of the CEO there are a lot of reasons to prefer a non closed source and non-gigant-tech-corp-controlled software forge.
Tests do not account for all possible executions of the code, rather only a subset of it.
Rust is indeed a safe language, in terms of memory safety. Vulnerabilities are still very possible within a rust program, they just need to not rely on memory exploits, and the borrow checker won't catch them. That is why formal verification exists. If you have a really critical, high security application then you should ensure the maximum amount of safety and reliability.
Formal verification enables the developer to write a mathematical proof that the program behaves correctly in all situations, something that the borrow checker cannot do.