Earlier quoted context omitted.
When Facebook wrote this, the tooling that already existed wasn't available to them -- ex-Google engineers at Facebook wanted something like Bazel/Blaze, but Google had yet to release that publicly. So this is a Hadoop situation where Google had this internal tool that someone else really wanted and because Google didn't open-source it they wrote their own version.
Isn't there a risk that reimplementing a previous employer's tool constitutes a legal/contractual violation of some kind?
It's almost certain that it is a legal violation and if a lawsuit was brought, I'd bet a million dollars in favour of Google; however, the tool itself isn't the final product and it's silly, counterproductive and just outright malicious to sabotage efforts at building good tooling in a competitor's company. Plus, it will set precedent and Google would have to vet every single new internal tool against what the previous employers of their employees were doing.