Before I answer, let me say that I'm personally anti-religion. I do however fully support freedom of religion as long as there is a 100% separation of church and state and doesn't hurt others. Your private religious practices are yours and should be protected.
I think in the context of the article, a "true" Muslim is one that's a fundamentalist. The Christian Bible has some absolutely horrible things and if you had a true fundamentalist Christian that was stoning adulterers and gays, not working on the Sabbath, etc., I don't they would co-exist in today's society. The difference is that there is nowhere in the world where you have fundamentalist Christians doing these things. You'll have the occasional "Evangelical" Christian that threatens an abortion clinic but that's it. The worst thing a modern fundamentalist Christian is going to do is pray for you to be saved because they are afraid you are going to Hell.
Meanwhile, there are plenty of fundamentalist Muslims in the world that imprison or execute gays, have child brides, have beheadings, sever hands and feet for crimes, forcing women at home, not allowing women to vote or drive, having women dress head to toe in clothing to prevent sin, blame the women if she gets raped and the list goes on and on. The thing is, this isn't a small minority. Granted, there are more moderate Muslims in the world than fundamentalists, there is still an uncomfortable number. What's disturbing is if you look at polls from France, Germany and Britain that has seen a rise in Muslim immigration, when there are questions on things like "Do you want Sharia Law" or "Should you go to jail or be killed if you criticize Islam or the prophet?", the numbers aren't 1-3%, they are 30% or higher.
If you are a true liberal or care about freedom and equality, this is a problem. Christians modernized and threw out a lot of their old beliefs. While they still monkey with politics, many of them look at things like the Old Testament as an allegory or something that fit 2000 years ago. I'm just hoping the same happens with Islam.