The three oldest occurrences in the Corpus of Historical American English are from 1947, 1952 and 1954 (see below). This therefore supports what the article claims.
However, as @Peter Shor's answer shows, the word is actually even older (from 1922 at least).
(...) up with the rest of the cows at milkin
time and he knowed she was prolly out
there on the gully some'eres with a
fresh-born calf, waitin for it to (...)
Harpers Magazine (194712) pages: 498-505 (1947)
(...) They say he wouldn't speak to Ma for a
month after. He'd prolly have
disinheritedme, seeing these, or shot
Mott for a Texas traitor.(...) "
Giant, Edna Ferber (1952)
I'm quite aware that you come from a
higher level of society. You prolly
made a debut and all that.
Play BadSeed, Maxwell Anderson (1954)
Note that as Google NGrams shows, the word gained popularity in the Internet age: