The assumption that the 700c wheel is faster than the 650b wheel is going to be with the same tire on each. The example I’m thinking of is the new Pathfinder Pro 47, which you can get for both 650b and 700c. The total diameter of a 700c wheel with a matching Pathfinder Pro 47 should be larger than a 650b with a 650b Pathfinder Pro 47 mounted on it. This speaks to the transition in mountain biking to 29" wheels which ultimately became the standard as the fastest.
Once you start changing up the type and size of tire and comparing different wheel sizes then who knows. For example is a 700c with a Pathfinder Pro 42 going to be faster than a 650b with a Pathfinder Pro 47 or vice versa? Because as you say the 47 on a 650b may be pretty close in actual diameter to a 42 on a 700c.
At gravel speeds I would think rolling resistance will trump aero gains by a pretty wide margin. There would likely be some losses with the 47 versus a 42, but I suspect pretty small.
Hopefully we will see some more data from Dylan Johnson and Silca as they are apparently spending a fair bit of time testing a lot of this. Dylan is riding his 47 Pathfinder Pro on 700c wheels. He trimmed the rear side knobs to improve clearance on his Factor gravel bike which is rated for 45c tires. I’ve decided to go in the same direction and put the Pathfinder Pro 47s on my G23 700c wheels. My Checkpoint is rated for 45c as well, so once I have them mounted I’ll decide whether to trim the knobs or not.