I signed into another phone's lightroom app and now ALL of their pictures synced to my cloud account

[WinandWoo]

Can this be undone?

[Elmira Pendleton]

Hello WinandWoo,

You could try going to "Recently Added" photos (displayed in dropdown for All Photos in your library), select all the photos added on that day from that phone and delete them. After you done that, to permanently delete them from your account, go to "Deleted" folder, select those photos there and delete them again.