The wiki doesn't have anything on the Vikings' preseason opener or Murray's snap count in it — that's not covered, so I can't confirm whether he got extended run.
What the wiki does tell us is that Kevin O'Connell signaled he'd settle the Murray vs. J.J. McCarthy decision during the first 1-2 weeks of training camp (late July), not extend the competition through preseason games (from the J.J. McCarthy and Kevin O'Connell pages). So the expectation heading into August was that the starter would essentially be locked in before meaningful preseason reps, with Murray the ~90% favorite per oddsmakers and the "heavy, heavy favorite" per beat writer Alec Lewis (from the Kyler Murray page).