Nothing, but what's the value when it's trivial to filter it away based on known good actors? There will be a frontline somewhere, if suddenly it's reporting drones on the other end of the country, and you know there is no good nodes there, you ignore it.
Spam specifically means (at least originally) so much unwanted traffic that it overwhelms legitimate traffic. So can I see why that commenter might have considered it a contradiction to say "the majority being truthful" as a strategy to deal with spam.
The name comes from a Monty Python sketch where "SPAM!" is being sung so loudly that it drowned out any other conservation.
I'd still say DOS is a little different because it's the server that can't tell what's valid; to the human, in that situation, if their request eventually completes then it's definitely their data coming back, it just completes slowly or not at all. With spam, it's a human wading through junk to find valid data.