alt Hacker NewsWhy that? In a very simple case, all services of a monorepo run on a single VM. Spin up new VM, deploy new code, verify, switch routing. Obviously, this doesn't work with humongous systems, but the idea can be expanded upon: make sure that components only communicate with compatible versions of other components. And don't break the database schema in a backward-incompatible way.
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The only way I could read their answer as being close to correct is if the clients they're referring to are not managed by the deployment.
But (in my mind) even a front end is going to get told it is out of date/unusable and needs to be upgraded when it next attempts to interact with the service, and, in my mind atleast, that means that it will have to upgrade, which isn't "atomic" in the strictest sense of the word, but it's as close as you're going to get.
So yes, in theory you can always deploys sets of compatible services, but it's not really workable in practice: you either need to deploy the world on every change, or you need to have complicated logic to determine which services are compatible with which deployment sets of other services.
There's a bigger problem though: in practice there's almost always a client that you don't control, and can't switch along with your services, e.g. an old frontend loaded by a user's browser.