When I stated that, I meant by standards of heroic mode fights. HP is a key stat for tanks in progression settings as the bosses hit like trains. (Trucks, trains, atomic bombs, whatever you prefer.)
HP and Stance Modifiers offset each other. You are no weaker (or stronger) against a Heroic raid boss that melees a tank with higher health for higher damage than yourself with lower health and lower damage (from all sources), as long as the offsets are the same.
Using it as a "key stat" depends entirely on the stance modifier you already have. A DK considers a different health level to measure himself comfortable against a raid boss' hits than a Feral does, so the Feral looking over at the DK's health is wasting his time.
You missed my point as well. HP, although not used in mitigation almost whatsoever, does serve the largest role of all the stats we have. Without it, we would die immediately. It doesn't matter right now if our hp is only 550k or as high as 700k. But in a fight similar to Madness, where the tank must soak a hit for say in this new tier, 950k damage, HP is necessary. That is all I'm saying. There are top tanks who put stam over other stats until a certain amount of hp, others do not. It is completely up to the tank.
My recommendation, however, is to get your mitigation stats up a bit. Hit and expertise both are considered priority stats until you have the 7.5% caps of them. It gives you more reliable rage generation, and gives you more powerful attacks to assist with some dps during boss fights. 2.3mil crits from execute on Elegon always help out during that last burn phase. Mastery, of course, is our top stat, as compared to parry and dodge. But of course, even that can be debatable, and an argument I'm not dipping into.
That's what I'd focus on. It has gotten me as far as I am now with no problems and it should do the same for you.