An F150 is also a poor choice, so I don't think of it as a point of comparison. As an approximation, the mass of any object is related to its embodied carbon, so smaller vehicles embody less of it. Massive vehicles embody current emissions and that is worth considering.
That only works if you take it at face value that buying either of them is a rational transportation choice, which I reject. Even if I accept the people need a weird truck-shaped thing with a useless 4.5-foot bed, a far better choice on emissions grounds would be the Ford Maverick XL, which has a battery 1% as massive as the R1T's battery, yet this tiny battery cuts the per-mile GHG emissions in half. The embodied carbon payback distance of an R1T versus a Maverick XL is over 100,000 miles.
My kid races mountain bikes so I have become extremely familiar with Rivian (and Cybertruck) MTB Dad, and I think they are a joke. With only a little planning I can get three bikes and three riders in a Honda Insight, while R1T Dad needs an optional accessory to get even one bike in the bed. People choosing these things are, 99% of the time, not behaving rationally. They are buying luxury goods that they believe signal their environmental credentials.
I don't find either truck a rational choice, but the car market can stay irrational longer than the climate can stay liquid.
The cultural irrationality of the truck/car market in the US crosses all ethnicities and class lines. If we are trying to evaluate the effectiveness of EVs, I think we need to compare the Rivian to the closest fossil fuel powered vehicle, even if it's something that causes me to disrespect the people making these choices.
