Most likely because it's BS.
I did some reading. The fort was being starved out. Lincoln offered to resupply with provisions, but not war .materiel. But some sources say that one of the resupplying ships was a gunship.
Will read more.
See? This is what I expect from you, and why I don't take you seriously. Rather than saying "most likely because it's BS", you should be finding the quotes from the various cabinet members that refute it.
Of course this point isn't something trivial like the origin of "sold down the river", so I can see why you have no interest in tracking it down. This point actually matters, and it goes against you, so you will put no work into it.