Downvoting is effectively voting for a comment to removed.

How so? Downvoted comments can still be seen.

When was the comment downvoted?

hmm, it does look like they did though.

looking at @mtgx's history, almost all comments have been flagged and downvoted until they're [dead]

No, the account was banned, therefore his comments are automatically dead unless vouched for. https://news.ycombinator.com/item?id=20587230

Interesting. I didn't realise that banned accounts could comment at all. I'll keep an eye out for that in the future before vouching - in retrospect it does seem like an account worth ignoring.