What I was talking about is something like this: SRE team has 1 member and productivity of 1, that person is overworked and if vacation is taken productivity goes to 0.

Hire second person, after onboarding productivity goes to about 2 (i.e. linear scale). Hire third person, productivity will probably be a little under 3 because now they need to spend time to be on the same page. Vacation is still shaky because with such small team, knowledge will be 100% siloed due to outside performance expectations ("hey you worked on X last time, I'm going to assign this ticket to you" repeated many times).

Eventually team grow to a point where they can handle all work load, share knowledge between each other and take vacations without fear (btw if you fear taking vacations - don't, it's not your fault if team can't handle without you).

You can think of "work load + process" as a data structure. If work load is bog and process requires a lot of synchronizations (every meeting is essentially a Mutex for the entire team) - you won't get linear productivity increase, instead you will increase lock contention.