Copyright protects you from market substitutions (e.g. someone taking your IP, and offering an alternative to your work). Being trained on your IP, it could certainly be argued that users would no longer need to purchase your book.

"Future competition" is a loosely worded way of saying this.

"It could be argued" but the judge in this case has already ruled that the training does not violate copyright. Market substitution only comes into play to determine fair use if copyright has already been infringed.