Prove of algorithms is possible as long as there's a known method of doing so in SMT. That means in practice, if someone has written a paper for formally proving an algorithm in SMT, you can mostly copy paste the proof.
zz is developed in parallel with a large project using zz and new syntax sugar features will surface slowly as they become practically useful.
That being said, it will never replace external formal verification with something like coq. They serve a different purpose.
I'm not involved, but one obvious answer is: broader portability than Rust (this is explicitly called out in the ZZ article). Clearly devguard is targetting a broader set of devices than the limited set Rust currently targets (x86; arm, mips, riscv in tier 2, with various caveats for bare metal targets).
zz is developed in parallel with a large project using zz and new syntax sugar features will surface slowly as they become practically useful.
That being said, it will never replace external formal verification with something like coq. They serve a different purpose.