Based on what I know about the structure of protobufs internally and without having looked deep into what UPB is doing... I'd guess it could probably be a stack machine that treats (byte)+ as opcodes. Most of the time I'd think of it as parser -> AST -> bytecode, but I think the "grammar" of protobufs would allow your parser to essentially emit terminals as they're parsed straight to the VM as instructions to execute.

In the couple days since I posted my confusion (threads merged or something) I consulted the daffy robots and figured out how it all works. Also had them come up with a design document for "a specialized compiler and virtual machine architecture for parsing Protocol Buffer messages that achieves significant performance improvements through a novel compilation pipeline combining protobuf-specific AST optimization, continuation-passing style transformations, and tail call interpreter execution."

Interesting times we live in...