They really can’t do that. I know it sounds “cool” to say that...but it’s virtually impossible.
Hey! What are you talking about! Which mainstream media outlet supplied you with that whopper. Do a little homework. There are no protections against forgery of postmarks that I am aware of. And why would there be? They are meant by the post office to cancel stamps... they really don't give a flying doodoo whether someone forges them or not. Many businesses lease machines that not only issue postage but print a postmark at the same time.
You can purchase a stamp that looks just like a normal postmark with any location and date that you prefer for about $15. Hell, a skilled ten year old with an exacto knife could probably make an acceptable looking postmark stamp out of a potato.
https://www.zazzle.com/postmark_create_your_own_rubber_stamp-256073791781290247