This is misleading. They made each of the different denominations of Christianity a separate group, but atheists, agnostics, and people who believe in a higher power but don't belong to any specific religion are all lumped together.
Go tell a Catholic and a Protestant that they'll the same religion and see what happens.
They tend to have a pretty big history.
And where do you want to draw the line for "same"?
Islam, Judaism, and Christianity all worship the same God.
The one that Abraham heard in his head that told him to kill his brother, then told him to kill his son but at the last second changed their mind.
They have minor disagreements on prophets and what food is allowed, but they're worshipping the same god a (likely schizophrenic) guy over 2,000 years ago said he could hear in his head.
In my personal experience, this really depends on the context. Most of the time what you say is true. However they are as opportunistic as anything else. If you are discussing things that point to the division of sects as a weakness, or how demographics don't stack up to other because of the division of sects (like in this article), suddenly they are perfectly fine with every other sect being the same religion.
It's misleading for certain purposes, but no purpose is implied by the headline. For some purposes it would be equally as misleading to categorise Mormons with Catholics. The denominations don't collectively act as a bloc on many issues/topics