I guess the question is how you (or more accurately, how does the site) define "rookie card" -- is it "first card/year by one of the major manufacturers in a major league uniform," or "first card released during/after their official MLB rookie season."
Not to throw another level on this, but the parallel, I'd think, would be players who come over from international leagues. For example, Ichiro has two cards (or years) with RC designations here: his 1993 BBM card, and all of his 2001 major US releases. Can they realistically all be considered rookie cards if there are eight years between them?
I see both sides of this, but I think like most things in this hobby, it's become somewhat relative because of the diversity of card sets and diversity of leagues. If I'm looking for a player's "rookie card" (quotes/emphasis mine in this case), I want their earliest card that's generally available to the public. For most players since 1948, most folks will consider that to be whatever year their first Topps/Bowman/Fleer/UD/Score/whatever card is, regardless of the set. It's why Kevin McReynolds, who I collect, has Fleer and Donruss RCs because he was in their 84 sets, but has no Topps RC because he didn't sign with them until 1987.
But to answer your initial question? It's a little weird to think of a guy like Johnny Mize, who debuted in 1936 and has cards in the TCDB going back that far, as having a 1948 "rookie card."
However the site decides, this would also impact other sports, no? https://www.tcdb.com/ViewSet.cfm/sid/72609/1981-Donruss