Something to consider is if aluminum was intrinsically superior why didn't the industry shift to it en masse? There are two reasons. One is that it's not superior. There are many materials that work well for driver cones. Aluminum is merely one of them. Every speaker manufacturer with any interest in it would have obtained free samples from driver manufacturers to find out for themselves if there was good reason to make the change. It would seem that they did not.
Second is that none of them would want to be seen as copying Hartke, in so doing tacitly acknowledging that Hartke was better.
There are a number of ways to configure drivers to give different results. Using aluminum cones is one of them, but hardly the only one, nor is aluminum used exclusively to realize better high frequency response. If that was the case why does this subwoofer driver have an aluminum cone? https://www.parts-express.com/Peerless-XXLS-P835016-10-Black-Aluminum-Cone-Subwoofer-4-Ohm-264-1648?quantity=100
Lastly, while Hartke seems to be the only major player that uses aluminum cones for electric bass, they're quite common in the hi-fi world. We're usually unaware of it, as most of them are painted black.