I can fill mine (bought in 2014) from the bottom with an octo. It has a duckbill valve at the bottom; a membrane that tapers inwards into a small hole.I know this is an old thread, but... I bought one of these in 2019 and finally dove with it this past month and really had not unrolled it until I went to rinse it and dry after my trip and noticed it is actually fully sealed at the bottom. Not the very bottom, but there's a clear membrane on the inside just past the mesh pocket (about 3-4" up from the bottom). It sounds like what you have will allow The current listing on DAN's site says you can inflate from the bottom with an octo, but the amount that mine is blocked off would be nowhere near enough to get it to go up. Were these sealed at some point to only allow inflation through the LP valve? I was wondering if maybe there was some sort of valve, but it seems really solid. I also tried and couldn't get air through with the octo in a tank.
I guess my question is were these able to be filled from the bottom with an octo, or is this just a feature of the newer versions. It sounds like you could do this back then from your description(s).
Perhaps yours stuck together while it was in storage? See if you can use a chopstick to gently pry the center open.