Theres a few differences tbh.
A player filling both a homegrown and an academy (club?) trained squad place is essential, a foreign player bought from another league is not.
Furthermore, academy players brought up through the ranks wont initially count towards a squad place. Inamoto was already 21 when he joined, so within the season would count, and for the duration of his contract would have had to be properly registered at some point.
Lastly, the pool of talent accross the world is incomprehensible when compared to the academy players available. Buying abroad is nothing like developing internally.
No matter what way you cut it; inamoto was different and was a failure.