Calling eachother at network level is still a dependency.
(And even a build dependency if you use something like protobuff or other protocol description files)
A network dependency is not a build dependency. Protobuf files should be copy pasted, not referenced directly. Saying you need a single repo to build correctly for your network dependencies is like saying you cant use a third party system (aws, etc.) Without having a link to their code base.
Point is that when you want to do a change in the "API" (or call it "protocol"), you need to touch the different repositories and coordinate to use the right versions together.
About the copy/paste of protobuf files, it works but makes it more difficult to keep them in sync.
And I did not say you need a single repo. I'm saying the stated disadvantages of multi repo are real.