That's confusing two different things, though.

A monorepo does make changes atomic in the code. There's no trap there.

You're talking about deployment, and yes when deployment is staggered, then obviously all atomic changes need to be backward-compatible, or else be very carefully orchestrated. But that doesn't have anything to do with monorepo vs polyrepo. That's just staggered deployment.

You have to deal with backwards compatibility in both cases. But at least with the monorepo you can see and track and merge all the changes related to a feature in one place, and be able to roll them back in one place.