Yes there are 8 international riders invited to race the keirin series each year. HOWEVER, they are invited to ride a special series for them and a number of selected Japanese riders, they do not ride in the general keirin series.
Those Japanese riders who ride against the internationals are not necessarily the best Japanese keirin riders. The best riders generally ride in the main circuit because that's where the bigger prizes are!
There may well be a change in attitudes from the Japanese riders regarding world/olympics in the next few years. Frederic Magne (from France, 3 times world keirin champ) has recently been appointed to the top coaching job in Japan on a long term contract (6 years?). He's had great success as track coach at the UCI sprint school in Aigle and insiders reckon he is already starting to make a difference over in Japan. Given that Japan has a huge pool of keirin riders for Magne to train and select from we could well be seeing Japanese riders starting to really challenge at world level in the next few years.
Will be interesting to see if it happens or not!