From my non-rider’s perspective, I think Black Mamba’s supposed similarity to Nemesis comes from the fact that it has a lot of trenches, and seemingly offers a very similar style of ride experience with the landscaping and near misses.
However, they seem as though their reasons for having the landscape are very different to one another, and the rides themselves are incredibly different track and train. Alton Towers had to opt for a somewhat unconventional layout when building Nemesis, because the local authorities wouldn’t have allowed them to build a conventional inverted coaster layout, thus the pit and the ride we have today were born due to some amazing creativity from John Wardley, Tussauds and B&M.
Phantasialand, on the other hand, built a more conventional inverted coaster in terms of track and train, but managed to emulate a similar style of ride experience to Nemesis through some very clever theming and landscaping.
I haven’t ridden Black Mamba, so please do correct me if I’m wrong on this, but that’s just how it looks from my perspective!