Won't work sadly, the "counter sound" can't be in phase in multiple locations because of delay and reflections. Headphones can do ANC because the ear is basically a single point so the mic can pick up exactly what you're about to hear and then compensate.

Using a speaker like that would just create double the leafblower noise.

Or leafblowers could include that as a mandatory feature

I mean, would they even hear anything over the leafblower?

Okay! I'll make sure to blast my sirens at my neighbour next time he leafblows. That totally won't get me in any trouble whatsoever lmao