This is a bit misleading to outsiders. Each of these repos was huge for the time, corresponded to a major subsystem with many disparate components, and the default tooling on the ground was the cross-repo tooling. One got the impression that if they could have pulled off one giant monorepo to rule them all, they would have, but they fell just short due to some technical details (cough spinning magnetic disks). In the meantime `sdx` was a convenient abstraction that allowed people to work in a monorepo way.

All in all it wasn't so different from present-day monorepos broken into git submodules for performance reasons.