It’s as secure as any C++ backend that performs no input validation.
Ie. whatever memory safety or sandbox you had from using wasm or rust is gone when you use it.
The llama.cpp author thinks security is "very low priority and almost unnecessary". https://github.com/ggerganov/llama.cpp/pull/651#pullrequestr... So I'm not sure why a sandbox would bundle llama.cpp and claim to be secure. They would need more evidence than this to make such a claim.