I'm not sure I agree with this. In this case, it is a noun, which references the actual judgement, and not the act of judging, so it alludes to the amount of time he believes to be accurate, but either way, the definition does not exactly present a condition that allows for a moderation of negativity, or "less-ness".
Conservative is actually an adjective ("conservative judgement") and an adverb ("conservative judging") first and foremost, though it can be noun-ized ("a conservative").
The judgement in this case pertains to the newness of the item. Hours of use are simply a measure of that property, and so a conservative judgement of the item's newness would equate to a higher number of hours of use, fitting the definition you posted exactly: a purposefully low estimation [of the item's newness] for the sake of caution.
Anyway, this sort of discussion is frowned upon in sales threads and just cluttering things up. : )