Lightdm

AFAIK they would get the info "which user said what". The source would be what ever instance you are from tho, not your own ip. So the owner of a instance knows as much as everyone else.

I think they meant your info being sold on dark web markets, not your behaviour on the dark web being sold.

I am not sure as well, but maybe they meant "maybe an early and public disclosure increases the urgency of the fix for the developers"?

"Elementary, my dear watson" is a sherlock holmes "quote", which is sometimes used to roughly say "it is actually quite simple". Take away the "my dear" and the post roughly sounds like the "quote".