Fine print is below, looks like Nissan will own the rights to anything you submit:
Grant of Usage Rights in Submissions:
By entering your Submission into the Innovation Garage, you grant to Nissan an irrevocable,
perpetual, transferable, non-exclusive, fully-paid, worldwide, license (sub-licensable
through multiple tiers) to (a) use, copy, transmit, distribute, reproduce, modify, create
derivative works, adapt, combine with other ideas or works, publish, translate, publicly perform,
and publicly display your Innovation Garage Submission (or any modification thereto), in
whole or in part, in any format or medium now known or later developed and (b) use (and
permit others to use) your Innovation Garage Submission in any manner and for any purpose
(including, without limitation, commercial purposes) that Nissan deems appropriate in its
sole discretion (including, without limitation, to incorporate your Innovation Garage Submission
or any modification thereto, in whole or in part, into any vehicle, technology, product, or service).
They own "a" right, not "the rights". At any rate, it means that they can implement any submitted idea without compensation or award to any person.
The idiocy with this agreement is that it says "fully paid", and without any renumeration actually getting paid to you, that test fails. Irrespective of that idiocy, even if "fully paid" is struck, it's irrevocable, so you could find yourself losing AND in a lawsuit with Nissan over the priority of rights, yours versus theirs.
You are correct in your first sentence, I am not a lawyer but interpreted the agreement as they are able to use/sell your idea or any modification of your idea while you still technically own your idea.
Grant of Usage Rights in Submissions:
By entering your Submission into the Innovation Garage, you grant to Nissan an irrevocable, perpetual, transferable, non-exclusive, fully-paid, worldwide, license (sub-licensable through multiple tiers) to (a) use, copy, transmit, distribute, reproduce, modify, create derivative works, adapt, combine with other ideas or works, publish, translate, publicly perform, and publicly display your Innovation Garage Submission (or any modification thereto), in whole or in part, in any format or medium now known or later developed and (b) use (and permit others to use) your Innovation Garage Submission in any manner and for any purpose (including, without limitation, commercial purposes) that Nissan deems appropriate in its sole discretion (including, without limitation, to incorporate your Innovation Garage Submission or any modification thereto, in whole or in part, into any vehicle, technology, product, or service).