The article mentions they have around 2000 engineers, at that scale investing in custom tooling can easily become a significant net gain in productivity and stability, especially when dealing with the JS ecosystem.
The fact that we can consider high-quality sourcemaps a solved problem (during both debugging and consolidated crash telemetry) really helps developers focus on building functionality rather than debugging a build system.
I think most large companies have dedicated tooling teams for these kinds of reasons. A related viewpoint in a related thread: https://www.reddit.com/r/typescript/comments/jrgi8z/10_insig...