I know that's a shocking headline, but that certainly seems to what they're saying for a certain group of users (red emphasis added):
By default, your System Photo Library is stored in the Pictures folder on your Mac, but you can move it to another location on your Mac or store it on an external storage device.
WARNING: If a Photos library is located on an external drive, don’t back up the drive using Time Machine. The permissions for your Photos library may conflict with those for the Time Machine backup
Jan 23 2018 update: Thanks to reader Brian for commenting below that Apple has updated this page with much clearer wording. It now reads (emphasis added):
If a Photos library is located on an external drive, don’t use Time Machine to store a backup on that external drive. The permissions for your Photos library may conflict with those for the Time Machine backup.
That just means you shouldn't use the same external drive for both your Photos library and as a destination drive in Time Machine. This makes much more sense; continue reading only if you care about my feelings on the original incorrect wording.