Brzezinski is not a historian but he is credible at least in this particular case. You can ask Carter he is still alive.
That's official history, In reality it was different.Both the US and USSR were vying for support of the monarchy, though that's not an invasion. The Saur Revolution and subsequent reforms after the coup produced a backlash of indigenous Mujahideen, which scared the Ruskies enough to invade. Once they did, the US saw an opportunity to arm the Mujahideen.
Did you read a book from Brzezinski?The counter-revolution was specifically driven by a desire to return to Islamic law and custom, and decidedly anti-Western - which is to say that the country may have had nice places to visit, but the countryside wasn't a terribly great place to live.
Every book I've read on the topic, both from American and non-American authors has the USSR invasion occurring before US involvement.
He's written a dozen or so. Cite the book and page where he states that the US had military involvement in Afghanistan before the Soviet invasion.