According to Serenity Caldwell (in the comments to the page below), the support page is poorly worded, and it is safe to delete the iPhoto Library.
http://www.imore.com/how-import-your-iphoto-library-photos-os-x#comment-form
That said, I have not tried it yet myself.
I think the actual meaning of that page is to say "everything has moved to your new Photos library, and the old iPhoto is just a shell of it's former self, but it points to the new library if you still want to view it. It takes up so little room now, though, that why bother...and you may want it later for some reason.
I think it would be one of the stupidest things ever to actually keep the original copies in the old iPhoto library, instead of moving them to the Photos library and then creating links in iPhoto. Apple does some pretty dumb things, but surely not that...