The comment just means you're not able to use the v14 release of the uSync package with an earlier Umbraco install. However there is some support for moving the data between the versions still. If you export your content from v13 uSync, copy the usync/v9 folder out and drop that folder back into the root of a fresh v14 project with v14 uSync, on first run the newer version of the package should then detect it as legacy data.

Initially it'll go into a 'legacy' tab for you to review and decide if you want to update it to the v14 format, which it then drops into a new v14 folder for you to import as normal. It's very experimental though and how successful the final import is probably largely dependent on how much you're using datatypes which have been deprecated/removed in Umbraco 14 versus what was in Umbraco 13. I'd also suggest sticking to Umbraco 14.1+ rather than the initial 14.0 release, as the bugfixes that came in with that can help reduce import errors.

Out of curiousity I just tried it on a v13 site I have here and managed to successfully get all the doctypes and datatypes imported and over 7000 content nodes, which is not an easy import to throw at it!