When you open it, it defaults to the "Photos" tab which has a message (not a dialog) asking for all photos access. But if you switch to any other tab (like Search or Library) you've got access to all of your photos in the cloud.
On the one hand you wouldn't realize that if you never tried tapping another tab. But on the other hand, the main "Photos" tab is designed to access all of your local photos and show whether or not they're backed up. Most (although not all) people are probably using Google Photos for this (to save local photos to the cloud), so it at least makes some sense for it to be the main tab with a prominent request.