You make good a good point inquiring about the "6mm rule" but, practically speaking, in this application it's not a significant concern.
Yes, it is true that the natural surface tension of glass in its liquid phase will tend to pull it towards a thickness of 6mm.
That process however is heavily modulated by, among other factors, the viscosity of the glass (dictated by process temperature) and surface drag and will take quite a long time to act on the large-ish sheets we're working with. You'll notice that about :30 into chapter two we mention that you should leave a 15mm allowance at the perimeter of the sheet to trim. This is because the contraction will have started there but because of the relatively brief period of heat-work at fusing temperatures, it doesn't cause very much distortion.
So, for the types of designs and firing schedules we show in this video, you can consider the final trimmed part sheets to be nominally 3mm.