[...]
Yoshida: When we make battle content, first we determine a minimum assumed item level required to clear. Then, we determine the total amount of party damage required to clear, taking a minimal amount of damage from tanks and healers into account during this calculation.
What do you mean by minimal?
Yoshida: Basically it's the damage from tank auto-attacks and the basic skill rotation used in order to maintain hate. We add this to the party damage needed. However, we often use zero in the calculation for damage required from healers. In other words, we only take into account the amount of damage necessary to perform as a tank or a healer and we don't take them into account as actively participating in attacking. For the clear assumption on DPS, we calculate the 100% value for the item level and we reduce that by 10 to 15 percent for the amount required to clear.
But it's often actually the case that healers do participate in attacking.
Yoshida:
Well, the calculations are based on the item level assumed during development.
Ah, and that's different from the minimum item level required to try the content.
Yoshida:
Yes. So if you try to clear it at a lower item level, it will be necessary for tanks and healers to participate in attacking. [...]
It's not 100% explicit, but it's easy to infer that he means "lower than the one assumed during development" in the last part, since an item level lower than is required to enter in the first place makes no sense, as that would mean you cannot enter. And since the two are explicitly different, that can only mean the assumed item level is higher than the minimum to enter. Which in turn also means healer DPS can be required to beat it. It's been chewed over numerous times.