It's hard to separate the two, and I think growing glosso horizontally indicates that most stem plants will grow perfectly fine. But you're right, it doesn't necessarly indicate that if all stem plants grow fine then glosso will grow horizontial.
I think the criteria for minimal lighting to grow "most stem plants" is define by the lighting required to get the best coloration and growth out of your stem plants i.e. Hottonia palustris, Ludwigia sp. 'Guinea', Blyxa japonica,
Limnophila aromatica, Alternanthera reineckii, etc. If the stated wattage also grows Glossostigma elatinoides horizontally then that's an extra bonus.
Hopefully, we as we grow through this process we can define low, medium, and high light aquariums a little better, and also narrow our definition above.