It's very easy for an arrogant scientist to say that it was stupid, but what was a better option? In our more enlightened era, we don't dispose of them at all, instead we keep shuttling them around. I'd argue that the waste is much better disposed of there than it isn't now.
Making an observation of an incontrovertible fact is not, in any way, "arrogant". And to suggest that no one had any idea that just sinking a shipload of incredibly toxic shit was a bad idea at the time is also "stupid". "They" knew. Most of those impacted did not, so "they" knew that they'd get away with it.