Just to be pedantic, this is not “official” in the sense that Disney does not officially endorse this. This is a grace period that, many have found, is built into the system. Its one and only purpose is to allow for some kind of flexibility: if your watch is a few minutes ahead, if there is a meal that takes longer than expected, if there is an unexpected delay in your previous ride, than, you will, smoothly, be able to enjoy your FP+ without even needing for CM assistance.
Disney will probably never publicly endorse this, because if they would, they would need another set of “hidden” rules to allow for the unexpected.
While most of the educated readers of this board know these rules exist, you should not count on them. If you do, be prepared to have your FP dropped if there is any unexpected additional event. For example, if you schedule going to a FP+ attraction 10 minutes late, but there is a 6 minutes line before the FP+ entrance, you will be 16 minutes late, and the CM may forbid you from riding.