They probably test at 70 mph because you wouldn't want to post a video premised on breaking the speed limit. However, the average speed on U.S. interstates is 78 mph, and power consumption rises non-linearly with speed. So, if you prefer to drive with the flow of traffic (which accident data show to be the safest speed, regardless of posted speed limit) or a tad faster, you're going to do considerably worse than a 70-mph range test will predict.
This is why we have tested both our Teslas ourselves with the cruise controls set to 80 mph. As both Teslas consistently read 2-3 mph faster than the GPS speed indicators in our radar detectors, we thus are setting the true speed at 78 mph. (This is another Tesla trick to make range seem greater. Our Honda speedometer conforms exactly to the GPS speed indicator in our radar detector.)
We do these tests on Alligator Alley, a flat, straight, lightly traveled stretch of I-75 through the Florida Everglades, in dry weather in temperatures between 80-90 degrees, and we use highway mile markers to gauge distance.
Our 2015 Model S P90D consumed about 10 miles of indicated range reduction for every 6 miles of actual road travel. The 2021 Plaid is considerably better at about 7 miles of actual road travel for every 10 miles of indicated range reduction. (Of course, what this is also testing is the accuracy of the cars' displays of remaining range.)
One of the things I'm worried about is the fact that Lucid uses no buffers in the 118-kWh battery pack, whereas almost every other EV manufacturer does. Unless they are using some secret cell chemistry that protects from deterioration at extreme high and low states of charge, it means they are sacrificing pack longevity to get maximal EPA range ratings. As an owner, I will plan every trip based on charging up to 90% and not letting the pack drop below 20% (just to leave margin for unexpected delays, inclement weather, or extreme topography en route).
All of this -- together with my preferred highway speed of around 80 mph -- means I will be assuming ~250 miles of real-world range in the Lucid Dream P on 21" wheels on a long road trip. However, that's still over 3 hours or driving, and more than I would want to do without a break.