How would anybody know that? Damn near every rookie QB struggles reading defenses. And it's not like anybody knows when Goff did and didn't have help reading defenses otherwise under McVay.
Also, Belichick's brilliant strategy wasn't to change the defense after 15 seconds. If it was that simple, we could have just run hurry up and snapped the ball before the play clock got to 15 seconds. His brilliant strategy was to crowd the LOS with bodies who could rush or drop into coverage to confuse our protections and put Goff in a situation where he was unable to tell who would rush or drop until after the snap. Unsurprisingly, that threw off our offensive rhythm and timing because Goff had to take an extra moment to figure out who was in coverage.
The thing is that defensive strategy was so brilliant that Belichick then used it to absolutely decimate NFL offenses for the entire first half of the 2019 season. Nobody had an answer for it until the Ravens in Week 9. In the first eight games of 2019, no team scored more than 14 points on New England's defense, and New England held five of eight opponents to 10 or less points. The Patriots finished the 2019 season with the #1 defense. It took the entire NFL half a season to figure out the defense Belichick used against us, and plenty of teams still struggled with it over the second half of the year.