It's close. The 35mm film diagonal is 43 mm but that should make little difference.
Isn’t the reasoning of the ‘diagonal’ argument to produce correct perspective, given the viewpoint? And isn’t that different from the ‘natural field of view’ argument?
I think a problem with the latter argument is that the eye and brain are capable of either taking in a wide scene or focussing on part of it. For me, those two modes are approximated by 35mm and 50mm focal lengths respectively. But for landscape neither really satisfies me. With the 35 I get far more foreground than I bargained for, and with the 50 I can’t fit in enough of the horizon. What I probably need is an Xpan, but I can’t afford one. 40mm is a compromise I can live with, but it
is a compromise. In practice I vacillate between 35 and 50 and take very few landscapes. With human subjects, I find field of view is dictated by circumstances, and perspective distortions bother me much less.