There is usually a lot of water residue in the tanks; it is never a good idea to fly on low fuel anyhow. My guess is that Lamia never drained their tanks routinely. That would be good engineering practice, so unlikely to have been performed with this company.
As for the clown in charge of this flight, and his actions, words fail me.
But to balance the anti-South American comments made here, I know of at least one very senior British training captain who routinely browbeat inexperienced FO's into operating with fuel well below plog. (Including myself, once, 11 years back). And this idiot was not even management, simply obsessed. Thankfully he retired recently. The mania for fuel savings and time saving can exist everywhere. It has no place in safe aviation.