You might be right about these not being sci-fi, but sci-fi can take place in the period in which it was written. Alternative history plus sci-fi can definitely be a thing. Or writing sci-fi that's supposed to take place in just a few years.
"Samuel R. Delany described it as “about as close to an SF-style alternate history novel as you can get.
Further, while it incorporates elements of the fugitive slave narrative, Blake’s narrator is also a scientist, whose focus on data collection and research stand in repudiation of the racial science of the day.[10] In fact, this reflects one of Delany’s major themes: that Africa and its contributions to science and math were foundational to the Western world.[12]”
Considering we’re talking about the era of the belief in Drapetomania, I’d say a slave revolt followed by an attempt by black people to take over Cuba would be considered sci-fi by a lot of readers.