Only if you spend the time to build tools to detect commits in your dependencies, as well as your dependent repositories, and figure out how to update and check them out on the appropriate builds.
So, no, it doesn't come for free.
You are totally correct that to achieve the same performance, correctness, and overall master level "green"ness in a multirepo system you would have to either define or detect dependencies and dependent repos, build the entire affected chain, and test the result. That part is much easier in monorepo.
What I was referring to with "this" is that Uber's method of detecting potential conflicts. In multirepo land it would be a "conflict" if two people commit to the same repo. In multirepo, therefore, detecting potential conflict is trivial.
If Bob commits to repo A and Sally commits to repo B, their commits can't result in a merge conflict. Well, unless the repos are circularly dependent - which would be bad :-) don't do that. Of course, monorepo makes that situation impossible so there's an advantage for monorepo.
It seems like whether you have mono- or multi- the problems solved by one choice will leave other problems the build system has to solve that it wouldn't have to solve if the other option were chosen.
Different work would be required in multirepo but it would be work to solve the problems that monorepo solves just by virtue of it being a monorepo.
You also would need to do that as an atomic operation (in the mono-repo + especially with a commit queue you're building on the atomicity of git).
Having to unwinding that transaction if you aren't atomic can get you into a big mess at larger scales.
Here's a good related talk: C++ as a "Live at Head" language: https://www.youtube.com/watch?v=tISy7EJQPzI by Titus Winters (from Google).
Currently, I'm not convinced that you need to track and apply commits across dependencies and dependent systems atomically/transactionally to have a sane build environment even at scale, but you definitely get that part free with monorepo.
Any links to docs or presentations that address that specific issue would be very welcome :-)
This holds true when A and B are leaf repos, but gets tricky with repos inside a dependency graph. More concretely, if C depends on both A and B, and it turns out that C depends on A and B in such a way that A_bob and B_sally are mutually incompatible, you need some kind of mechanism for reconciling that.
Of course, exactly as you point out, mono and multi are two tradeoffs for the problems that large codebases intrinsically are.
Introduce a breaking change into a common library and now you have to update every other dependency to support it.
Not so bad in a monorepo. But when your codebases are distributed?
There's always a window where both will be in use, because we can't synchronously replace every running process everywhere (not that it's even a good idea without a canary). The shorter you try to make that window, the more needless pain is created and plans disrupted. While we could use prod to beta test every single version of everything, that shouldn't be our priority.
In this case you'll have a very short transition, between all consumers updating their client code (possibly in a backwards compatible way) and the change in the implicated system being deployed, not the other way around.