GCD heals did not stack. But oGCD still does, which what made the comp viable, although wasn't optimal back then. Earthly Star? Opposition? Essential? Overpowered Noct Intersection? Yeah they all do. Neutral was also there. SCH/SGE do not see much of an issue today because they're both mitigative & HPS monster with the latter having literally every single piece of their action littered with questionable heals even when they're not properly used. (Pan)Haima & Holos are probably the more egregious examples. Noct AST back then only had Helios & Neutral to fall back on should they needed to pump out heal when their SCH mate was down for whatever reason.
Also my copy pasta comment was referring to how SGE is quite literally a carbon copy of SCH minus opportunity costs. The only similar part about Noct AST and today's SGE is literally only their fully mobile ST GCD shield.
EDIT: to be clear, my comment to Percibel was mostly me lamenting how pointless SE's attempt to split the healers in Regen/Barrier dichotomy. If Noct AST stayed throughout 6.0 - today, there really won't be any harm at all. Most people would probably still go Diurnal. Nocturnal probably won't be optimal, but still viable, which is my point. It's not broken, and some did enjoyed that 'playstyle'. The only few QoL they needed was to ensure only stronger barrier remained intact & maybe try to build a way for ASTs to dance between sects with added risks, if the player want to.
When SGE shipped out and people still able to pair both barriers together, albeit by designating who to prioritize GCD shielding (aka the same way as old Noct AST + SCH comp did in the past), it begs the question... then why do we even remove Noct AST at all if that same problem just jumps ship to today's SGE? (and in addition to that, being a carbon copy of an existing job instead of something fresh in that new design space... but that's a rambling for another thread) All of this just screams "Noct AST was removed for absolutely no reason".