(File>Consolidate) as he believed there could be photos where the originals were in a different folder.
If you are using referenced photos (stored outside the Photos Library), these photos will not be syncing with iCloud and remain only local in your Photos Library. But they do not prevent the other photos from syncing.The status message below the "Photos" album will list the number of items that are local only, and the number of items that synced successfully. referenced photos do not interfere with the iCloud syncing, they are not recommended for other reasons.
I tested with two referenced photos - and the status message looks like this: The two referenced photos are staying put on the Mac, and the other photos sync.

What does the status message below the photos show in your library, if you select the "Photos" view with the Moments?
If the status message is saying "uploading ... items" it would indicate, that you are having files in your library that are not in a standard photo format. This is possible, if you migrated an iphoto library or Aperture library to to Photos. For example, my Photos Library contained a few PICT files and audio files, because they were in the iPhoto Library I migrated. If there are critical files, you will be seeing a message like this: