Historically they used to update individually but as Sonos is a multi room music system designed to be left on, this means when there is an update it’s now done to all speakers simultaneously even if there is not an update required on some devices.

It takes about five mins or so per speaker (including reboots and to be fully running). A Sonos home theatre setup would then take about 20 mins alone which is not acceptable, in my house I would be fuming with updates taking about an hour if each Sonos device was done individually.