I think the headline for the news article is somewhat misleading:
Atheist 'mega-churches' take root across US, world
It's probably a handful of people. And they are hardly "mega-churches".
Atheists gathering together for some talk and music is all well and good.
But I suspect a few will wonder: what's the point?
What's the point of coming together? What is it are they marking?
With Catholics and many other churches, we're marking the salvific sacrifice of Jesus Christ on the cross. We are also partaking of Communion, the Sacrament that flows from that sacrifice.
Many Protestants have foregone going to Church altogether: there's nothing to mark. They can pray at home. They can sing at home.
Atheists can listen to talks at home. They can listen to music at home.
Community is important. But what is this community based on? What binds them together? A rejection of religion? That's not enough to hold people together.
I suspect that this movement will divide into its ideological camps. Some will be open to God. Some will be closed. Some will consider themselves a religion, others not. Some will even try to proseltyze.
I think that the main thing that's missing from this effort is meaning in life. There's no coherent message of the purpose of human existence. I realize that they will say "to create one's own meaning". Except we don't create ourselves. We want something bigger than ourselves. That's ultimately why people go to church.