Programs written for one type of computer cannot work with another.
In order to make it do anything, you had to write a program.
"How long would it have taken for someone to write a program to bring down the whole system?"
Anyone can write programs for it and eventually, some of those will not work very well.
It was therefore possible to write programs which would run on all 3000 systems.
The task is to write a program which says for all programs whether they will finally stop.
Joe wrote a little program that he put into Bill's big computer.
And so, many early programs were written for them specifically.
You don't even have to write a program that runs.
So he wrote a program to take advantage of this.