QGIS successfully opened the file for me so I guess there may be ways to export to a format that M9 can use
QGIS used GDAL to open the file (QGIS has no native ability to read GeoJSON). If you don't mind using GDAL to open the file, Manifold can do exactly the same thing as QGIS and use GDAL as well.
Both QGIS and Manifold can use GDAL. It's just that because QGIS has no native dataport capability you're stuck using the third party GDAL package, while with Manifold you get hundreds of native dataports built in, with no need to mess with GDAL in most work. But if you prefer GDAL for any reason, no problem, it's there, and it's nice to have GDAL to connect to niche formats or niche uses, like the politically correct but spectacularly inefficient notion of publishing gigabytes of vector data in a (you can't make this stuff up...) text format. It could have been worse, of course. They might have picked braille, or even worse, GML.
To my taste, the best part of GDAL is OGR, specifically the ogr2ogr mega-utility that is an easy way to mass convert files, if you don't mind using a command prompt and dealing with a program that has about 800 command line options. That's worth it just to scratch the nostalgia itch, if you miss command line work in CP/M, DOS, or UNIX.
A better format than GeoJSON to use overall for larger vector data sets would be GPKG, which you can easily create from the .geojson using ogr2ogr. Launch the following in a command prompt within the folder where the file is located:
Convert all .geojson files in a folder to .gpkg:
for %f in (*.geojson) do ogr2ogr -f GPKG "%~converted.gpkg" "%f"
Convert one .geojson file:
ogr2ogr -f GPKG "Australia.gpkg" "Australia.geojson"
Still waiting for my Australia GeoJSON file to download, so I haven't tried the above, but it looks right (to a non-expert in OGR...).