This was fixed a while back in one of the main rulebook updates:
https://www.warhammer-community.com/wp-content/uploads/2019/05/warhammer_40000_rulebook_en.pdf Page 248 – Ruins
Change the first paragraph of rules text to read:
‘Unless they can Fly, Vehicles, Monsters, Cavalry
and Bikers can only be set up or end their move on the
ground floor of ruins.’
Emphasis mine, on the words added by the errata. The original wording in the rulebook was simply that they could not end their move there, hence people parking tanks on top of buildings like you describe.