Def not serious
Like any information it's best to pull from all sources and use reasonable deduction skills from your own play. Personally I've found that in order of good information it goes Discord > Icyveins > MMO Champ forums = Wowhead guides. Also learning how to understand logs helps as well.
However if you are a casual raider Icyveins probably all you need. Just remember that all the guides are written by different people. So one spec might be better then another and vice versa. It's why people have such mixed opinions about the site.
Icy veins is complete garbage for some classes, however. For example, the advice for balance isn't even accurate for any of the fights in TOS. The discord channels have the best resources, trinket priorities, etc. They also suggest simming, but you also have to look at the builds folks are using on each fight, since Sims obviously aren't going to accurately predict all encounters. And sometimes it also relies on your raid doing specific things to maximize certain class outputs. Like keeping the second bosses close enough together so that aoe taps both of them.
Did you forget how stupid easy KT was in Naxx and how stupid hard his slow ass circles were for people? I would have thought the hard part for people would have been the thinking on the fly for the last phases, but then again I wasnt there for the 96 wipes.
Never got to do Naxx in vanilla beyond a couple of the earlier fights. Was a bitch getting people attuned and player retention towards the end of things was terrible. All I remember from KT in Wrath was being bored out of my mind and pulling extra mobs to pad the meters.