The way I'm reading this is, MS realized that, since the FS2020 will start with "clean slate", no legacy file formats and such, it will not be able to be fully and transparently backwards compatible. So, they decided to continue on with FSX (and probably bring it in line with current standards) so as not to crash the current add-on industry and a wealth of developed knowledge base.
I personally couldn't be more ecstatic and happy about this announcement. While I've developed my add-on (FSX SpacePort) with modularity and portability from the get-go and I always expected I would be upgrading it one day for FS2020, this news gives my product (and obviously any other FSX add on) an extended longevity and a new lease on life!!
Also, my personal experience with DTG was dismal. I had a video chat about my add on with them more than a year ago, and they were excited about it, but since then, they fell silent, not responding to my correspondence. I have my product on Steam, but its release is in limbo, since it hinges on DTG blessing (Steam and DTG have (or, I guess now, "had") a sweetheart deal where developers needed a DTG permission in order to sell their content on Steam). I hope all that changes with this announcement.