I think after their shambles of a car last year, they went ultra conservative and wanted to get the platform for the future, have a car that can be built upon. It wouldn't surprise me if this year's car was made for the Honda engine.
That would be the hope, but I can't help but feel that McLaren as a whole has been stretched slightly and a bit distracted with all that effort in building a road car business. They just haven't had the experience to be on top of everything yet.