This news needs to be tempered with other facts. The traditional church affiliation cultural norm does not apply today. Some churches don’t have an “official roll” of church members/attendees. I know the Calvary Chapel churches don’t.
This could also be a pairing down of nominal members who aren’t Christians to begin with. When Christianity was not only culturally acceptable, but expected in many social circles, you belonged to a church or you were socially stymied. That is not the case any more.
Perhaps the enduring, more successful churches today are the ones who are serious about their faith, not social clubs.
That’s why we have AOC and Illian what’s her face.
My guess is that many of the "non-members" may be more sincerely following Jesus than many in the heretic mainline denominations they have left. While migrating to seek a community of deeper faith, their search may likely involve studying the Bible more than someone would who is a complacent "social Christian" who is in a denomination mainly due to family history. Seekers would also be studying the creeds of the churches they sample to find out where the Holy Spirit would have them land.
"Those who come to me cannot be my disciples unless they love me more than they love father and mother, wife and children, brothers and sisters, and themselves as well." Luke 14:26 (Good News Translation)