It's been there since the beginning, but I disagree. Aspected Benefic and Helios have always been footnotes in AST's healing identity. What I think of instead is Earthly Star's delay and timing, Essential Dignity's low% HP mechanic, The ability to extend regen effects by 3 to 5 ticks (which did nothing for shields by the by).
Those have always been the memorable parts of AST's kit. Changes in ShB have brought the sects more into general usage (CO/CI), but that's not what I think when someone says 'AST healing identity'
Could it be that AST feeling like a WHM clone is actually because WHM itself is a boring cardboard cutout baseline of a healer instead of DiuAST being similar? Because their only similarity is their GCD skills.
Don't get me wrong, WHM works in its current state but its skills are almost entirely just +HP or +HP over time. The more interesting skills being assize, which the on-cooldown demand makes rather dull, and Asylum, whose healing buff could make for interesting teamwork potential but its high potency results in basically just another +HP skill.
If anything, AST's raw healing tools of Earthly Star and Essential Dignity, with their more engaging mechanics, make for more an evolution of WHM's concept than any skills attributed to WHM itself.
Is it really AST's fault for being 'samey' with WHM, when WHM boasts a base kit and little more?
The skills not stacking won't force Nocturnal. Nocturnal fails because shields aren't necessary in 90% of encounters, especially with allies who avoid avoidable damage.
If you don't need shields, then having them is worthless. So the sect that boasts higher healing numbers wins.
Nocturnal can't be forced unless shields are forced, and forced shields are eventually circumvented with tank cooldowns and gear progression. AST just needs a singular healing kit, with some regens and some shields, and no bogus 'dichotomy' shoehorned in.
The same with WHM, and SCH, and the next healer.