It is always partially true, but each city is different. People who travel in the very early hours (like 2:30am) always have problems, (even the best cities run reduced and thus inconvenient service for maintenance reasons, most give up on transit completely). While those hours are not common it is safe to bet everyone reading this has had reason to travel at those times at least one night in their life.
In far too many cities, (and not just in the US) additional service is run during the peaks. By additional service I mean they run more buses/trains as opposed something with more seats (that is longer). This means people who travel at non-peak hours have to be careful about when they travel to ensure there is service without waiting. Of course if the wait is still less than 5 minutes nobody cares, but as waits get longer - humans don't have time for that.
> most public transit currently doesn’t serve those people well because it’s focused on work-related commuting patterns.
Does it? Work-related travel is mostly during morning and late afternoon, but public transit also runs outside those hours. In my experience public transit works quite well for non-commuting travel. Of course this can vary wildly by country, but the claim that public transit only really serves commuters is not universally true.