I do not think that's true. Permissions to delete a file/subfolder depend on permissions of its parent folder (which you didn't tell us anything about), not on permission of the file/subfolder itself.

Though anyway, this is not a question on WinSCP. As at more appropriate place, like https://unix.stackexchange.com/

I was able to delete a directory that had sticky bit set with the directory owned by another account. I thought this was not possible. Please advise.

WinSCP Version 5.9.5 (Build 7441)