Well you can't actually use that argument cause they do take tank dps into account when designing the encounters. That's actually in the quote. Did you read the full quote?
But the point is in no way does any healer ever need to dps in any circumstance. Yes clearly if you can do it it helps, that's pretty obvious and nobody ever debated that. People are trying to force the idea that it's necessary, or healers are bad if they don't, and that is simply not true. Trying to make other healers feel bad because you want to play at a top level is not the best way to encourage people. Even Yoshi stated the following:
Yoshi doesn't want people to feel like they have to and doesn't want the top tier, world first players to make others feel it's necessary either. The fact that in one short quote he emphasized twice (technically three times now that I reread it) that healers do not need to dps should be all you need. Any healer that wants to dps while also performing their other duties at a top level is going to be doing more yes. But healers that don't dps much or at all when it's not otherwise easy to do so (aka they are standing around doing nothing with plenty of mana, clearly that's a bad healer), should not be called "bad" for not doing something the designers are not expecting them to do. They are good healers doing what's expected of their job and doing it well. This sort of toxic attitude you guys are putting out is really detrimental to the community. Instead of putting people down for merely trying to meet expectations to the best of their ability and telling them they're not good enough, try something more positive and less hostile.Certainly for people who are at world's first level, their goal is to clear it at as low an item level as possible, lower than the one assumed during development. So if you look at the fight and figure out that if it's not numerically possible to clear with four DPS and tanks, you'll need to make up the gap with DPS from healers. Then when those publish clear videos and other people see the healers DPSing, they might think that healers need to be DPSing even though its a situation that only arose because their clear would have otherwise been impossible.