...he said the people playing clave on those few recordings just didn't know and played it wrong,
Goyo used to tell me the same thing, "they didn't know what they were doing back then", but to me it was always an unsatisfactory answer - it's not just a "few" recordings we are talking about, it's more like dozens, some of them by groups under the direction of Alberto Zayas, Ignacio Piñeiro, and Odilio Urfé - and there is no consistency even within those recordings by the same group. The change to (and codification of) playing the open tones on the 2 side coincides with the advent of very popular recordings by Los Papines and Guaguancó Matancero in the late '50s and early '60s.
My personal belief is that, prior to those recordings, people just didn't care that much about how it was played. Only after those newer recordings reached a wider audience, and especially after the formation of the Conjunto Folklórico Nacional, which established a "correct" way to play many types of folkloric musics, did the open tones on the 2-side become standard.