It doesn't say they are Nigerian pirates, just that the sailors were kidnapped off of the Nigerian coast. Muslim or any other pirates do not observe borders. Muslim and pirate are pretty much synonymous now.
I don’t KNOW they were Nigerian Muslim pirates. But this is a long, long way from the Somali pirates. And there is a history of Muslim piracy off the Nigerian coast. That part of the country was mostly Christian, back in the days when there was fairly constant warfare between the two sides. And the Muslims were annoyed that the Christian Nigerians were getting most of the oil money. So, there was considerable violence and piracy off the coast, by Muslims.
I’d say the odds are at least 95% that Muslims were involved in this one. Because that’s the history; and also, that’s what Muslims do.