Snagx files are replacement for the previously used .snag and .snagproj files in effort to create a single file that works across all versions of Snagit. In this sense snagx to jpg conversion describes simple export to JPEG format.
You should be able to open and export .snagx file with any modern version of SnagIt, but it is also the only software that can be used for such export. We do not know about any 3rd party solutions that could be otherwise used for such export.
Updated: January 8, 2024