You do not need to scrub or write anything to not provide user A’s data to user B in a multi-tenant environment. Sparse allocation can easily return nulls to a reader even while the underlying block storage still contains the old data.

They were just incompetent.

On top of all of that, when I pointed out that what they were doing was absolute amateur hour clownshoes, they oscillated between telling me it was a design decision working as intended (and that it was fine for me to publicize it), and that I was an irresponsible discloser by sharing a vulnerability.

Then they made a blog post lying about how they hadn’t leaked data when they had.

Nope.