That's the problem with the word 'picky' - it can mean different things, and different things to different people. I wouldn't have considered the Splinter Rifle thing to be picky, certainly not unsporting. If people don't know a weapon's statline then they don't know it, can't really fault them for that. I don't have a problem filling them in if they ask. Mind you, if they asked such questions constantly throughout the game then I'd get pissed off, it would be like they thought I was playing in bad faith. I also wouldn't consider something like 'Can Skimmers emerge from a surrounded WWP?' to be a picky question either, but some people probably would. A picky rules question to me would be a chancer trying to weasel advantages without any real basis, but some people would probably disagree with me here.
Just seems like far too loose a term to be using. Different judges and players are going to interpret the term in different ways. Some players might end up afraid to ask judges for rulings on the grounds that they might be seen to be being 'picky'. Some might ask perfectly reasonable questions and end up getting flak from the opponent for being 'picky', while another opponent would have answered without batting an eyelid.
Some people might even think that asking what 'picky' means is a picky question in and of itself... But honestly, I really don't know what GW mean by the term. Would have helped if they'd been a bit more verbose. They should have gotten Cryx to write the fugging things!