Yes, it is just there to provide as flat a surface as possible. Your idea of the glass sheet is a really good one. Like you suggest, as long as you can place it in a way that it can't rock around due to any irregularities, it should work great. It might actually be better than the resin in some ways. I'm waiting to see how the resin holds up over time. While I painted the base completely, with many layers, it is still a wooden base and can potentially move around if changing humidity levels can reach the wood. And the epoxy would move with it.
I also like the glass idea because it reminds me of a technique that Dave Gingery used in his "Make your own Metalworking Shop from Scrap" books. If I recall correctly, he attached two plates of glass to pieces of plywood by bedding them in auto body filler. He then lapped the two plates together to create a super low budget surface plate. I remember ordering those books when I was about 12 years old. I haven't looked at them in decades. Good stuff.