The article is entirely about tooling for safely changing wheels mid-journey. In that context, it's not weird to expect the database to remain available during updates.
Yes, it will require more work and time than just taking the database down and performing the update while it is offline. But as long as the database remains available it doesn't really matter if the update takes 5 minutes or 2 days, just that you can do changes faster than they appear. Since DBMS updates happen at most every few months, that should hopefully not be a problem.
At one of my previous workplaces we had a multi-TB table that could take several days to migrate with the online tooling, and would take 12+ hours to migrate even offline. Nobody wanted to take 12+ hours of downtime (for a busy customer-oriented website) but as long as the db stayed up nobody much cared how long it took.
