As a Yank, I'll be the first to admit the crew in this case made the primary error. "When in Rome, do as the Romans do - however weird." Not dead sure you understand the clearance? STOP - and get it clarified
before you take off.
I'm not really thrilled about "fly heading two hundred" or "flight level two hundred" - smacks of contacting aircraft G-GEDY as "Geddy" instead of "Golf Echo Delta Yankee."
But in some places we're already on that path - "United two four six", or "United two forty-six."