The information I was looking for, what kind of interpreter they mean, was in an Ignition article linked from the main article: https://v8.dev/blog/ignition-interpreter

"With Ignition, V8 compiles JavaScript functions to a concise bytecode, which is between 50% to 25% the size of the equivalent baseline machine code. This bytecode is then executed by a high-performance interpreter which yields execution speeds on real-world websites close to those of code generated by V8’s existing baseline compiler."