Stable driver API and open source/GPL'ed drivers are orthogonal.
They are both significant for some companies in deciding whether to support Linux, but they are independent from each other. If Linux ever had a stable driver API, the GPL requirement would still cause some vendors to not provide drivers for Linux. If Linux dropped the GPL requirement (such as it is), but did not promise a stable driver API, some vendors would still choose not to provide drivers for Linux.
Main difference to Linux: stable driver API. So vendors could make their blobs and support them easier, without open sourcing, like linux demands.