I think he means stateless, at some point a system needs to have some internal state to keep things running and that state can be really hard to manage when it is mixed with all other functions in the system that also have state.

Even if that state is kept outside the service itself (like a database or event queue) it can still be really hard to reason about when said state-stores are shared across a huge codebase. Changes to a part of the state can have negative effects in completely unrelated functionality.

And of course, there is nothing blocking a monolith from isolating/modularizing their state-stores, but it tends to not happen unless the architecture forces it to happen or through strong tech leadership.