Expectations should be lower as far as responsiveness or even availability, for someone who is not actually on call. The load (and expectation) is also not evenly distributed: IME senior and staff/principal-level engineers (and managers) tend to get paged in when off-call much more frequently, for obvious reasons. It's more likely to be "I need someone who knows XYZ", not "I need absolutely EVERYONE" https://www.youtube.com/watch?v=74BzSTQCl_c or "I need a random additional pair of competent hands".

Also, IME it's been relatively rare for issues outside of business hours to require calling in people who aren't really on call. I think the article is pointing out that it can be the right thing, not that it's necessarily a common scenario. And during business hours, being pulled away from your other work to help handle an incident is obviously a much easier pill to swallow.