“Therefore replication is *intrinsically* possible, given the right conditions.”
That’s why they prevent it.
I don’t understand how you can be so confused about this.
There are two ways people use the word impossible.
1) Intrinsically impossible, like in the movie Team America: World Police, where the guy asks, "Have you ever seen a man eat his own head?"
2) Practically impossible, "There's no WAY the Minnesota Twins are going to win the World Series this year." (But they did in 1991 when they were given 1000-to-1 odds against it at the beginning of the season.)
It is not intrinsically impossible for replication to occur in a vaccine: all you have to do is screw up the processing of some of the ingredients going into it.