That is correct. Opening up the threads means that the prefit barrel you purchased will likely not fit if the thread tenon has been cut for a factory action internal thread diameter.
Most places will take 10 thousands or so to open up the threads. As a result, the tenon of the prefit will be too small.
You can also talk to the prefit manufacturer and ask if they can go with a bigger tenon, or alternatively, you can ship your action to them and they can fit a barrel for you.
There are other considerations, but most of the time, this is the case.