They are talking about the accessibility of the media player controls, not the accessibility of the media file.
As such, I am inclined to agree as long as the native file is in a 'sensible' format that is well supported. It is possible to get free media players for most common file formats for most operating systems, albeit that users may well not have these installed, so it would be necessary to provide a link from which they can be downloaded.
It would still be necessary for the media files to have captions and audio descriptions where appropriate.
Although this solution would be technically compliant, in practice we often find that people do not know the keyboard shortcuts for their native media player. Of course there is an onus on them to learn how to use their machine but I suspect that most people are used to playing audio and video through their browser, not the native media player.
