Or, they've been given crap primitives to work with. There's only so much lipstick you can put on a pig. I don't know what database they're using or what their pub sub and streaming looks like, or even what their system diagram actually looks like. But, well, you don't see Google having these kinds of problems. Other ones, sure, but between Chubby and Spanner, if Google had bought GitHub we wouldn't be having these problems.
But it wasn't a pig. It was a reliable system, and then it increasingly became an unreliable one, in a way that is not explainable by the mere increase in demand. Whatever rearchitecture was performed, it was done and is apparently being perpetuated by software engineers who should be held accountable. Not necessarily guilty, or even directly at fault, but accountable nevertheless. "I am just an employee of a bad company" is not a valid excuse for an engineer.