As I understand it, it’s just as they said:
Calculating primes is fairly straightforward so you calculate a few large prime numbers, and do some math to them.
Now you have a strong key that didn’t require a supercomputer to create but taking that final number and turning it back into those original primes is a much more computationally expensive proposition.
In fact, it’s one that’s not viable with current technology.
Sure but they couldn’t prove criminal intent so it’s ok.
/s