Not all longifolium are hincksianum
ALL hincksianum are longifolium.
Phrag hincksianum is a geographic race of the wide spread species longifolium. (think hincksianum is a subset or subspecies)
Phrag longifolium includes the wide leaved forms from Colombia & Venezuela that can have flowers stems 5 feet tall down to the little Ecuadorian races with narrow sedge shaped leaves and total less than 12 inches tall. Hincksianum is the Panama race of longifolium with narrow sedge shape leaves and is about 24 inches tall.
My taxonomic recollection has not been updated to the 21st century - I have not read much taxonomy the last 5 years so my info is a little out of date. I believe there is an article out there citing why hincksianum is not a valid name, also gracile, and proposing the correct names, with a clearer definition of what constitutes what - but I have not read it yet - so I don't know the latest. But the name hincksianum was in common use when I was reading the literature - so my comments reflect the way the name has been used through to 2001.
What is really key - these names are tied to geographic regions of origin. If you have a plant, and you don't know where it came from, you may NEVER be able to say with certainty what the plant is beyond calling it longifolium. The small forms of Phrag longifolium from Ecuador commonly called the variety gracile are difficult to differentiate from the somewhat taller growing form from Panama - but the vouchered speciemens truely known to be from one location or the other are definitely different subspecies, and clearly not of the same origin. Once they get mixed up in cultivation there is no way to separate them reliably. There is no vouchered DNA referance database that can be used. One would have to go through the expense of going to each country and collecting DNA there in order to create a reliable vouchered DNA database. Unfortunately the money is not available to do this. Unfortunately what people tell you about the origin of any particular plant in cultivation may be suspect - if there were any mis-recollections or mis-statements as the plant changes hands, you can end up holding a mis-labelled plant.
To my eye, your photo and description of a plant whose vegetation is only 6 inches tall more closely suggests the subspecies gracile which is of Ecuadorian origin, rather than the taller hincksianum which is of Panamanian origin.
But a badly grown hicnksianum might be shorter than a well grown gracile. With plants in cultivation it is often impossible to say anything more specific than that's a nice Phrag. or maybe that's a nice Phrag longifolium.
