If you Google "Titanic coal fire", the results page will be full of articles positing this exact theory. This is neither new, nor correct. Anyone who has studied this event at any length will know it was a death by a thousand paper cuts. Trying to assign ultimate responsibility to a coal fire using a 106-year-old photo showing a black smudge is laughable and shows a fundamental lack of understanding.
The fire in question wasn’t some blazing inferno that many undoubtedly imagine, but rather it was more of a smoldering affair. This fire was in one of many rooms that was built for the express purpose of containing this coal. A fuel source that is known to self-ignite. To say that the fire effected the ship is like saying burning a casserole in your oven will burn down your house. Possible? Technically. Likely? Absolutely not.
Something this article failed to mention was that the fire was put out the day before the Titanic sank. Coal fires were common on large steel ships like the Titanic. The engineers and stokers were prepared for, and ultimately handled the fire. It was part of their job, not some apocalyptic emergency.
As I said above, the Titanic was a death by a thousand paper cuts. Unfortunately, it was a necessary death to give us the marine safety regulations we have today. If any number of things had been just slightly different on the Titanic, it would be a footnote in history and we would be talking about a different ship that would have inexplicably had everything go wrong, including but not limited to an irrelevant coal fire.