The article talked about a trend, which would require an earlier survey to compare the recent one to. Young people aren’t given any voice in any church’s statement of faith, so whether or not the statement is unchanging is irrelevant. Saying that young people believed in them more in the past than the current batch of young people would require an additional data set.
A church's statement of faith is as meaningless as a not-followed company policy.
Additionally, the survey polled individual Christians, not churches relative to their belief statement.
The survey notes the trend in differences in beliefs between the historic Christian faith and younger Christians (there is an earlier survey and Barna has related data going back decades). The data shows a clear trend away from the historic Christian faith and biblical world view.