Difference is, your team can more easily make up the difference if you're missing DPS because the healer is not able to DPS as much, compared to non-healer healing.
I don't know, and I don't think it matters. The fact of the matter is that the first DID do it, and THAT is the problem. It shouldn't have been doable in the first place. The fact it was, shows there's a design failure that needs fixing. The fact it was done for TEA, DSR and now TOP, not only shows that the design failure has persisted for over two years, but TOP removing both healers instead of just one means the design is going further in the wrong direction.
Imagine an Ultimate gets cleared, on content (remember, we're on patch 6.35, TOP came out 6.31), by a team of any composition of your choice, as long as no member of the 8 is any kind of a DPS. 4/4 tank/heal, 2/6, 1/7, doesn't matter, just no DPS. Would that be fine, that the enrage check is so lax that it's possible to clear without any DPS at all? Of course not. I don't know for sure, having paid very little attention to the minutiae of the fight, but I'm under the impression that you're almost hardlocked into having 2 melee because the melee LB is so important in the final DPS check. Additionally, you're hard-locked into a physranged and a caster because you need to have those LBs. Yet somehow, the 'you need Healer LB for this' mechanic was bested, and a group did not in fact need healer LB. Or a healer at all. So rather than asking 'oh how many other groups did a run like this after the first', ask 'why was it possible for a group to even do this'