There are a few faults in the point you are trying to make.
HOTs tick once every 3 seconds. That is more than 1 GCD per tick. Burst damage typically comes in very fast, all at once, and typically does not even bleed into a 3rd GCD. So, we are looking at -possibly- two ticks of a HOT, depending on where you are at on the server tick. Secondly on this specific point, you are assuming you had ample time to throw up both aspected spells to get the stacking HOT, which is not always possible, especially at times of high pressure.
While I won’t claim to know the exact intricacies of how the server applies damage and healing, I can say with certainty that damage and healing are not applied in successive order. Countless times, heals go off before damage and you die anyway. While I have no concrete insight as to why this is, I think most would agree that this game favors the attacker due to the nature of how numbers are applied.
Repeat casts of Noct AB is -not- irrelevant -at all-. Being able to spam a potent shield on yourself with an instant cast spell is extremely powerful and one of the core strengths of the Astrologian. In your given example, you are assuming that you are able to freely cast Benefic 2...which is not the cast in which Noct stance shines. Nobody here is suggesting that you should be spamming Noct AB while you are able to cast Benefic 2. What is being suggested is that the Noct AB is extremely potent for snap healing while you are under pressure and otherwise unable to cast Benefic 2 freely. Obviously you can use Lightspeed and Swiftcast to make it instant, but you don’t always have those tools available.
You disregard another core strength of Noct AST (and shields in general) in that they raise your effective maximum HP. I cannot stress enough how powerful this is in The Feast when most jobs can kill someone 100-0. HOTs are not going to save you from a melee LB or Between the Eyes, but a shield might.
I’d also like to state something for clarity: I am not saying, nor have I ever said, that Noct AST is more powerful than Diurnal. Diurnal is, in fact, more useful in more situations. However, Noct AST still has very potent core strengths that are extremely powerful in certain situations against certain compositions.