There are a couple ways they could be doing this.
They could be melting/mixing it all together in a furnace (much hotter than a kiln) and creating a new glass that has its own COE. That is how most glass is recycled.
I have also seen work that is made by crushing different glasses to fine frit sizes, mixing them, and re-fusing. The net effect is a piece of glass loaded with stress – but in such “bits” that there isn’t sufficient force in any one direction to result in breakage.
As an aside, the real benefit from recycling glass comes less from the glass not making it the landfill and more from the significantly reduced energy needs to melt raw ingredients (batch) if there is cullet (recycled glass) in the mix.
Helios Kiln Glass Studio