This isn't with the front seats, especially the front passenger seat, all the way forward. This video from Dominic Wilkerson aka @starbux (which follows the longer video from Starbux posted earlier in the thread by @R2dreamer), provides clarification on the the measurements we were after.
In the original thread it was said it is with seats full forward and there are other videos with a Rivian staffer who is 6'4" only fitting diagonally and somebody 6'2" same FWIW.
Unfortunately it turns out they weren't good sources of useful information. The thread from SXSW was confusing to me because seats were shown in different positions in different images/video and there wasn't clarity on exactly what conditions applied when the demonstration was done, which is why I wanted a definitive measurement. That R2 was also fitted with Hest mattresses so anything less than about 78" was inconsistent with the Hest mattress measurement data.
