You can't kite buggy in Ft. DeSoto, is my understanding. I am surprised you saw another buggy in there.
If in doubt, just go into the ranger station and ask for Jim Wilson. He's a nice guy, and will give you the most accurate answer.

Probably the best local spot for kite buggying is Treasure Island.
They have a kitebuggy and kite club, Treasure Island Sport Kite Klub, that has been meeting out there every Sunday for practically ever.

I think that might be the only local beach that buggying is allowed on.
They worked out an agreement with TI and have an area that they buggy in.
It's a nice, large area. It's best after it rains, 'cuz the sand gets compacted.
You will be able to meet a bunch of local buggiers out there. Here is a link.

http://www.tiskk.org/