poe2 Icestorm: Is the 0.15s hit cooldown per storm instance or shared between all active storms?

I've been trying to understand exactly how Icestorm's hit mechanics work, and I'm having trouble reconciling the skill description with the existence of multiple sources of Icestorm limit.

According to the skill tooltip, Icestorm states:

"Each enemy can only be hit once every 0.15 seconds."

At first glance, this seems straightforward, but I'm not sure whether this restriction is applied per Icestorm instance or globally across all active Icestorms.

To explain what I mean:

Suppose I cast Icestorm multiple times and have several storms overlapping on a large stationary boss.

For example:

1 active Icestorm overlapping the boss
3 active Icestorms overlapping the boss
6 active Icestorms overlapping the boss

What exactly happens in this situation?

Interpretation #1: Per-storm cooldown

Each individual Icestorm has its own 0.15-second hit restriction.

In that case:

Storm A can hit once every 0.15s
Storm B can also hit once every 0.15s
Storm C can also hit once every 0.15s

This would mean overlapping storms can all damage the same target independently, resulting in increased single-target DPS as more storms overlap.

This interpretation would also explain why the game has multiple ways to increase the Icestorm limit.

Interpretation #2: Shared global cooldown

All active Icestorms share the same 0.15-second hit restriction against a target.

In that case:

Storm A hits the boss
For the next 0.15 seconds, Storms B, C, D, E, and F cannot damage that boss

If this is how it works, then increasing the Icestorm limit would provide very little single-target damage gain and would mostly serve as an area coverage mechanic.

The reason I'm confused is because the game seems to strongly support increasing Icestorm limit:

Icestorm starts with a base limit of 3
Stormweaver can increase the limit by +2
Certain amulets can increase the limit by another +1

This allows reaching a total limit of 6 active Icestorms.

If overlapping storms cannot independently damage the same target, then these limit increases seem surprisingly weak for boss damage.

I've also noticed that Path of Building appears to increase total DPS as the number of active Icestorms increases, which suggests that overlapping storms are expected to contribute additional damage.

However, I'm not sure whether PoB is accurately modeling the actual in-game mechanics or simply assuming each active storm contributes damage independently.

Has anyone tested this in-game against large stationary bosses?

Has GGG ever clarified whether the 0.15-second hit restriction is:

Per Icestorm instance
Or shared across all active Icestorms

Any testing results, developer comments, or datamined information would be greatly appreciated.