Cullet is actually a variant of word collet. This refers to the ring of glass that was left on the blow pipe when a glass blower was finished with a piece. The glass blower usually broke the leftovers off the pipe with a small hammer. The pieces were called the collets and recycled in the next batch of glass. Nowadays cullet means most any scrap glass.
Flint glass usually refers to glass with a high lead content. But lead is not used much in regular glass container production anymore. Most flint glass nowadays is made with Soda and Lime (EAPG). The term flint can be applied to most any hard clean glass for recycling. Plate glass etc.
Hope this helps.