That doesn't prove that this is real, but think twice before trying it out....

The safe way of avoiding that is to inject it directly into the FR24 feed, rather than broadcasting it as ADS-B.

A considerable proportion of their data is receiving from end-users running their own receivers, rather than the FR24-branded receiver. They do reject spurious data from one receiver but genuine-looking data would be hard to detect and discard.

https://www.youtube.com/watch?v=CXv1j3GbgLk

I wonder if this is a potential security risk, considering ADS-B is not only used for flight tracking but also air traffic control and collision avoidance?

Could a bad actor (using a drone, perhaps) broadcast a false signal and trick aircraft into performing collision avoidance manoeuvres? Or flood the ADS-B bandwidth with false signals to confuse ATC?

EDIT: https://www.flightradar24.com/how-it-works#mlat

So it's not that it couldn't be done or is inherently insecure, it's that a choice was made (either by omission or by intent) to not do it.

Note that anyone with a hundred bucks can buy a used aviation voice radio and do all the spoofing they want. It just isn't a problem because people don't like jail. The problem of getting acceptable reliability in an aviation electronics environment is tough enough that deliberate attacks fall below the noise.