The game containing public domain images wouldn’t make the entire game public domain. Someone with a copy of the game could distribute those particular assets though. Maybe. It depends on how much human effort was involved; an AI image can become copyrightable if enough effort was done to transform it after it was generated.
It depends on what you are using placeholder assets for. If you want to use it to gauge how a scene would look before setting out to build it, then placeholders that stand out get in the way. You would need a way of tracking all the slop, but then you could have a build tool track how much slop is still in the game to make sure you catch it all before release.
Their license also says they will not let anyone use their logo. So it doesn’t appear to be a reasonable attribution.