Is there a specific reason to doubt this particular article?
I think we can safely accept the account of what happened, though we can't really know what motivated these men. We know Mobile authorized a local Creole defense force in late 1862, confined to the city and county of Mobile, and we know this force was formed because they were turned down for Confederate service in 1863. So there was a non-white military presence in Mobile for several years, which could be a factor in black men ready to join up. However without any real information as to why they responded as they did, we can only speculate.