OP is correct.

I did the math on this myself when I was deciding on whether to play WHM or AST last tier and while I can't remember my exact findings, it came out quite similar to what was just posted above. And with AST's being regens which you don't need a lot of the time, plus the fact that WHM can do Plenary -> Lily x2 for an on-demand 1200 potency and Horoscope isn't as strong potency-wise as Plenary in that way because you don't always want to buff it with a GCD heal, so you're more likely to get 400 value from a Plenary than 400 from a Horoscope.

Of course AST can still do some great things with its tools which don't require the AST to be in range when they remotely detonate, but I think WHM has been stronger since the Lily Buff (it wasn't before) and I don't think anyone has talked about it because people don't really analyze healer potency numbers in this way even on The Balance or w/e and have been used to AST being considered the stronger one for a long time. But the Lily timer change was huge.