The airwaves are considered a public space so it's not two private parties and the rules are different. (You can't just use air raid sirens to disrupt a public protest.) However, at a minimum you are likely to run into issues with the computer fraud and abuse act because your interfering with two other parties, and that has rather stiff penalties.
PS: Random noise is one thing; hacking someone’s Wi-Fi even using such a simplistic approach is very different.
It's two private parties acting in a public space. It may still be illegal to interfere, but the first amendment protects you from the government, not from other individuals.
You can't just use air raid sirens to disrupt a public protest
I'm not 100% up on my civil rights/protest law, but if you are a private party, I suspect you could. It is, for example, 100% legal to surround a protest with other people holding sheets to block them & their signs from view.
Of note: A violation of a noise ordinance is in most jurisdictions not considered a disturbance of the peace unless the perpetrator has disregarded an affirmative request that he or she reduce the noise to a reasonable level. However, that would be implied in the air raid siren case.
Anyway, I agree it's not the 4th that's the issue. But, it's still unlawful conduct.
Talking over each other would be the same as running two or ten routers on the same channel, which is not an issue. This approach actively prevents communication.
PS: Random noise is one thing; hacking someone’s Wi-Fi even using such a simplistic approach is very different.