How does cullet reduce the production temperature in glass production?
No expert here, but cullet would react faster than sand with the alkali, developing into a more uniform fluid that can be mixed at a (slightly) lower temperature so as to eventually dissolve the sand. 

It is like beginning the reaction to produce glass at 25% (or however much cullet is used), rather than 0%, so that raising the temperature is not needed to to speed up the reaction. Lower temperatures can be used without needing to increase the time, and thus wear and tear on the equipment can be reduced.

So it's not that the cullet causes a reduction in temperature, but rather allows it.