The intent with that term was to imply that your 2 tanks both have to be in tank stance for the majority (65% or more) of the fight. Otherwise, the dmg is too much. They also would have to make proper use of CDs. So, lots of tank busters that can take 75% of a tank's HP (without CDs), requiring good use of CDs to handle. A tank buster every 60 seconds or so, with moderate dmg every 10 seconds. The 20% dmg red/25% hp boost would basically required to handle this properly, because CDs alone wouldn't be enough. Then, any decent tank would probably also carry fending right side for this fight as well. Kind of how the good BRDs would carry 1 or 2 more pieces of accuracy gear, just for specific fights -- but would tend to only use that gear on those fights. In other words, if you don't want your healer to kill you because you made his/her life difficult during these kind of fights, even the World First guilds would have their tanks use tank stance a lot, have 3-4 Fending right side pieces equipped, and use CDs well.
Of course, eventually, folks would outgear the encounters (say 6 months down the line); but so what? We're talking about designing fights for a certain ilvl, not about designing fights for future proofing.
Why not? I'm not talking about requiring you to NEED vit gear in dungeons or normal raids or even in Normal/Hard Trials (possibly not even in any new Extreme Trials) to tank those. (Note that we don't currently NEED str gear in those instances either; it just happens that many experienced tanks outgear those encounters in their str gear anyway.) I'm talking about designing certain "Savage Raid" fights such that carrying 2 extra pieces of Fending gear would be very useful. (This would mean that, on those specific fights, good tanks would have 3-4 Fending right side pieces equipped.) It's not as if most tanks don't do their current end game gear progressions like so:
1. Get ilvl160 tank gear as much as possible.
2. Buy as much ilvl170 tank gear as you can, while working on step 1.
3. Upgrade ilvl170 gear to ilvl180, while also starting to run Alex Normal.
4. Replace any ilvl180 gear with ilvl190 gear and/or ilvl200 gear. This is where tanks tend to grab 2-4 Slaying right side pieces.
5.a. Make sure you have as much ilvl200 gear as possible, while also keeping any ilvl190 BiS gear.
5.b. (Optional) Work on Alexander Savage with your static group.
If we simply adjust a bit so that we take into account the new Voidark gear (coming in 3.1) and any 3.2 gear, then it's a slight adjustment for the potential "new meta".
In summary, 1-2 fights that really test both tank's ability to be TANKY could easily be designed with this kind of concept in mind.