Healing is a form of damage mitigation. Getting health back is very important and if it can be stacked with mitigations the health returned is also more valuable, so the flat percentage is frequently applied to the HP regained as well, meaning yes, the mitigation does scale off gear as well as long as there is a heal attached to the mitigation. It only does not scale with gear if the mitigation either doesn't provide damage reduction after the fact or doesn't provide an HP return during the damage reduction duration. Anyone who says healing is not a form of damage mitigation needs their tank que times revoked.
To be clear, a heal that doesn't overheal and a shield that protect before damage are only distinguishable from eachother in terms of mitigation when the shield provided actually prevents death. Otherwise they are, in effect, the same value if the heal is the same. Heals are only weaker because they happen after damage is taken rather than before, but there's not a single mechanic where a shield is necessary to survive it in the first place, so it's redundant, and even if it was necessary, we have SCH and SGE for it anyway. This means in any sutuation that brought, as an example, Heart of Corrundum to the point the heal activates during the mitigation period and didn't outright kill the tank, then there is no situation where HoC alone didn't mitigate more than TBN even if you stacked it with Oblation. This is especially apparent on tank busters.