I don't think you really want to argue about what Townsend Whelen did or did not know.
He shows a chronograph in his book. And given that he lists muzzle velocity AND velocity at 100yds, I think he knows very well what muzzle velocity is referring to.
PS: The book metadata shows 1918 as the publishing date, but this copy has a model 1920 in 250-3000 in it, so I think the date needs to be revised to 1920.