Some important context to this 2018 article is given here: https://www.twilio.com/en-us/blog/archive/2018/introducing-c...

TL;DR they have a highly partitioned job database, where a job is a delivery of a specific event to a specific destination, and each partition is acted upon by at-most-one worker at a time, so lock contention is only at the infrastructure level.

In that context, each worker can handle a similar balanced workload between destinations, with a fraction of production traffic, so a monorepo makes all the sense in the world.

IMO it speaks to the way in which microservices can be a way to enforce good boundaries between teams... but the drawbacks are significant, and a cross-team review process for API changes and extensions can be equally effective and enable simplified architectures that sidestep many distributed-system problems at scale.