Kind of.

Not because it would be impossible, although the "every phone" is a bit of a stretch given how hard it would be to build an exploit that reliably works on all the messed up versions of Android that vendors put out.

But because if you had a capability like this, you wouldn't burn several full exploit chains just to broadcast a speech.

Doing something on the network side (either compromising existing infrastructure, simply being the infrastructure provider, or providing fake base stations) and then simply calling each phone - sure.

Pushing emergency alert cell broadcast messages with a link to the stream - sure.

Actually exploiting the phones? Nah.

This. How can you know you hacked EVERY phone. I have friends with flip phones (no screen). There must be at least a small amount of optimistic thinking here.

IMHO someone was likely given a task such as "disseminate the message to 100% of the population" and they found a way to claim they accomplished the task (with reasonable credibility).