If I understand correctly, Flawless provides the WASM runtime. If that’s the case why can’t it be entirely hidden to the user? The system provides things like entropy and networking.
like "just" WASM as a target doesn't have many of the APIs build in without non trivial "magic" trics
and for WASI might be tricky to get the exact form of determinism they choose right
like they might need to provide a custom standard library which is viable but not very convenient to use and even less convenient to maintain (if it's just about custom build flags for the "upstream" standard library it's on the other hand quite easy but likely not good enough)
through that's purely speculative
but they might be lucky there is quite a bit of benefit for rust to have a deterministic wasm build target (for wasm based derives which if combined with cryptographic hashing of inputs form a potential building block for having a shared (potential public) derive binary artifact cache speeding up CI systems and similar and allowing more complicated/advanced derive usage without having to worry about the derive compliation time adding to the initial build time)