Yes, the bottom pane is held together to the sides by silicone, just like all the seams. It sounds like your concern is that the bottom panel could fall downward without support, stressing the bottom seam. My counter argument to that would be: that cannot happen, because the bottom panel is resting on the rim, and the rim is resting on the edge of the stand. So the edges of the bottom panel have nowhere to go.
I'm talking about any rimmed, floating bottom, glass tank up to a reasonable size...say, about 300 gallons.
Thicker panes help. At a certain point, some things don't scale up. I understand that. It makes sense to go acrylic, for example.