General ideas for game mechanics vs the ideas for a movie or a plot for a game (where we start getting into Intellectual Property Law) are slightly different, Mog.
The person who submitted their idea to Square probably involved certain FF characters, a plot, etc. We're just giving general feedback (such as ideas to improve game playability, enjoyment, etc...) that doesn't necessarily jump into the purview of IP Law. Basically, we're not really creating the story or plot behind FFXIV. I suspect the person involved in that letter was, though not necessarily about XIV itself.
There's probably a better way to phrase this, but as I am not really an expert on Intellectual Property, that is pretty much the best way I can explain it.