I had noticed the same article about the Anglican Church (Church of England) in the UK from an unrelated source. The entire Anglican Communion (which includes the Episcopal Church in the US) has been undergoing a lot of turmoil for the past 10 years or so - many members have been lost, and others have been disillusioned with the Church. So I'm actually not too surprised by this, though it happened somewhat quicker than I think anyone expected.
What the quote glosses over and does not say, is that overall church attendance in the UK is considerably higher than mosque attendance. Just the Roman Catholics alone have nearly as many weekly churchgoers as the Church of England and the Muslims combined. And that doesn't count the many Christian denominations such as the Presbyterians who are neither Catholic nor Church of England.
So his quote of the article is, at best, selective and tendentious; at worst completely biased and misleading.