You would have to move your private photos to a different Photos Library so they will not accidentally appear. Hiding photos is meant to make the moments easier to manage, but not to keep photos hidden from preying eyes.
See the Help page:
You can hide selected photos and show them only when you want to see them. Hiding a photo can be useful if you don’t want to use it, but you’re not sure you want to delete it.
- Hide photos: Select the photo or photos, choose Image > Hide [number] Photos, then click Hide Photos.The selected photos disappear from your moments, but are not deleted.
- Show hidden photos: Click Albums in the toolbar, then choose View > Show Hidden Photo Album.The Hidden Photos album appears among your other albums. To unhide photos in it, select them, then choose Image > Show Photo.
Moments are created automatically, and we have no way to remove a photo from a moment, other than deleting it. Hiding will remove photos temporarily, so we can weed out the moments without deleting the photos.
But the hidden photos will show in any album. You could use smart albums with the rule "Photo is not hidden instead of standard albums.