Hm. Could damage prevented through percent damage reduction considered to be a form of healing?

The only reason why flat damage reduction gets equated to healing on these graphs is to give you some equivalent measure of a shield healer's performance. But while you can draw analogies between HP, healing, and mitigation and try to relate the three, the function of each one of these three things is distinct and there are circumstances where they should be treated differently.

The most obvious place where flat damage reduction and healing play out differently is if you're soloing something. If you have healing that can keep up with the incoming damage on average, then you can continue to fight indefinitely. You can't do the same with flat damage reduction unless you prevent all incoming damage indefinitely (i.e. you're permanently invincible), as your health will eventually chip down to zero. Likewise, if you have something that could instantly kill you in a single hit, healing doesn't help but flat damage reduction does.

If you want to compare like with like, then it should be to compare percent damage mitigation with flat damage mitigation. There are still some differences, of course (as the amount mitigated by %DR scales up with bigger hits), but it's a fairer comparison.