I'm not 100% sure but if it means just like how normal stable releases are then it means that the update will be offered to devices randoml starting from the day of release until usually within a week. This is done so not all devices will get the device at once and there will be a chance to stop the release and mitigate issues in case there's critical regression found in the update.
In focal's case, I would assume this will only affect devices that are already in focal since switching has to be manually done for now. Honestly, thinking about it, this might be a statement standard in every OTA blog that maybe shouldn't be included in this particular one since it's the first stable release for focal so no one's really in that channel yet 😅