Warren was certainly more familiar with the guts of MQL than anybody else, but PathQuery was designed for Google. PathQuery was actually the second system he built for extracting protobufs from graphs. He was also a longtime functional languages aficionado and the prototype implementation of PathQuery was written in Haskell. It took many smart-person-hours to get Warren's interpreter to work at Google scale and speed.
I think what differentiates PathQuery from many "query languages" is that it takes on the task of transforming that data into somebody else's schema (aka "turning protos into other protos"). Having a DSL for this is particularly attractive in a company that will otherwise expect you to do string formatting in C++. But even under normal circumstances there are wins from having data transformation vertically integrated with your query language.
I think what differentiates PathQuery from many "query languages" is that it takes on the task of transforming that data into somebody else's schema (aka "turning protos into other protos"). Having a DSL for this is particularly attractive in a company that will otherwise expect you to do string formatting in C++. But even under normal circumstances there are wins from having data transformation vertically integrated with your query language.