Consider the 10.y.z simply to be 0.y.z and everything works out.
Jellyfin inherited a lot of shitty code and architecture from emby. They simply cannot guarantee anything across patches until it is sorted out.
imho much better then releasing major version after major version because the break stuff regularly.
If you read through the whole paragraph, it is clear that they mean the compatibility of previous jellyfin versions.
Also, again:
That means that the code is not cleaned up with that release.
If you would release 11 before the code is considered cleaned up, you would basically break your own defined versioning convention. That is best decided by the active maintainers.