And there was a transition period where younger people need to know that the term could be ambiguous. It's not like they burned all the maths texts in 1974 and replaced them with newer literature. Older text books were probably in use into the 80s and perhaps the 90s.
From the writing of the era, it's clear this new definition for million was not popular, and many chose to continue using the "British meaning." So it was probably in colloquial use for quite a while, and the transitional term "1 thousand million" became the proper style.
