There is a significant difference between having a supported defined interface boundary, and a semi-stable interface that doesn't necessarily guarantee backwards comparability to prior interfaces.
Which is literally the delineating mark between an API and and ABI.
A stable ABI means your binary interface will remain the same and your compiled drivers should work, despite any changes on the kernel level. A stable API means you'll probably have to recompile your drivers when the kernel changes, but that they'll require little (if any) modifications.
Of course I’m not implying that the driver interfaces have a guarantee or will not change, especially with the rolling release model of Linux. However, you can be pretty certain within a major point release that there won’t be significant breaking changes. Thus the “pretty stable”. It’s not a pro nor a con, it’s just the style that works for that open source project.