When using the new Manage Ads option in the Ads Manager you can select experiences you do not own, as can be seen below:
I am not the owner, nor have any affiliation with these games, however they appear as options. You can even choose and add thumbnails to the advertisement, which is a risk If you upload anything innapropriate.
This is further confirmed by the response when fetching the universes:
Expected behavior
I would not be able to select or advertise games that I do not have edit permissions in any way.
Page URL: https://advertise.roblox.com/create
A private message is associated with this bug report