The word 'tank' means different things to different people. People who play other MMOs use it because of the way some games play, most DDO people tend to just call any melee a tank now, and the 'main' thanks is the guy fighting Sulo, everyone else is the 'off tanks'. And considering the ability of many two weapon types to be able to have a high AC AND have a higher dps than some old fighter with a shield say - the terms tank and dps really are pretty fuzzy in DDO anyway.
There are only a couple quests/raids in the game where having the 'optimal tank' matters at all really.
In 95% of the rest of the game, someone who is an 'off tank' - as defined by having the numbers to be able to tank the Vod boss - can excell, and possibly even solo most quests as the 'main tank'.
Or...people can show their snobbishness at every build that cant tank the single raid they might care about, and make fun of 95% of the players builds that dont fit their elite standards.
Basically anyone who plays the game regularly knows what people mean when they talk about an off tank. Posting about it in a snobby, deriding tone only serves to salve someones ego and try and prove they are better apparently because they dont have 'off tanks'.
I am guessing someone had one or more bad experiences with bad players, or bad builds trying to do something they couldnt, and decided to come air their grievances about it on the forums.