This is it exactly, pure simple logic, tried and tested many times. It matters not the specific special mechanics an LRM has if any... Massed LRMs are hilariously broken.
No, it demonstrates that PD is not well-balanced vs. LRMs and Macross spam, in general, because most PD for SS mods and Vanilla were balanced for a world of very finite, bursty missile spam.
This is why Exigency was generally very broken vs. Vanilla the last time I looked at it, balance-wise (which was like a year ago, so it doesn't reflect on current balance, mind ye).
Now that that is not necessarily the case, PD in SS should reflect that fact of life, but it generally doesn't.
To put it another way, none of those ships being targeted were using PD that was remotely efficient against those tactics; AOE PD really comes into its own vs. missile spam.
Burst-fire beams should indeed have a serious problem with spam, as they're one-target weapon; they have a definite saturation point, but should be balanced so that they're superior vs. individual high-power missiles.