Nobody has completed the iCP course yet, so no, that's not the reason the ATP isn't open much.
Remember that the ATP scheme is open to anyone with a CPL/IR, so there are literally thousands of people trying to get through the door of that scheme. Whilst you may be given preferential treatment if you do well on the iCP course, it is in their best interest to take the best of the best. Don't be fooled by thinking it'll be a simply thing to get on. At the end of the day, business is business and they want the best.
That said, if you do well, prove them wrong about not being suitable for the Wings scheme, then there is a good chance you could get on to the ATP scheme. Bear in mind that the ATP scheme is rarely open. So if you ace the exams and flying program, you could well be good enough to get into the ATP Scheme, but due to the fact it probably won't be open, you could be waiting 6-8-10 months for it to open.
At the moment, it's been closed for about 6 months. In my opinion, view the iCP course as any other integrated course. Don't get caught up in the possibility of attending the ATP course as a sole reason for picking the course - least of all when it hasn't happened to anybody yet.
Good luck!