I think that statement means that they have added a pregnancy system for various monsters in the game, however only 1 enemy is able to impregnate the MC as of right now. So, the 'tech' is in place for pregnancy for various monsters, but it has only been enabled/implemented for 1 enemy.
There is a slime monster in the beginning of the game that can get the character pregnant, you have to wait for them to knock the MC on their back and then wait to be captured by the slime. There is another slime pregnancy later in the game, there is a map where you are in a 'haunted house' style area and there are some slime patches you can get caught in, that can impregnate the MC.