'Alien' bettas are hybrids. I'm not sure what the exact cross is (seems like B. imbellis and B. mahachai are most commonly used), but they're certainly not pure wild bettas, and should never be advertised as such, no matter how close to the real thing they may phenotypically be.
This is why I am 100% against betta hybrids, no matter how beautiful they may be. Hybridisation in the wild is one of the driving forces of extinction for splendens complex species, and I hate to see people taking fish from the wild and using them to produce hybrids in captivity. All it takes is for these hybrids to be mislabelled as pure, and you can ruin the purity of captive bred fish.
Alien bettas and any of the splendens complex species can be just as aggressive, if not more so, than the ornamental forms. If experience with bettas has taught me anything, you can't make blanket statements when it comes to aggression and these fish. Some individuals will tolerate living in breeding pairs, or even groups, long-term, while others will attack or even kill their breeding partner.
Considering their wild heritage, I'd keep an Alien betta like I do my wild bettas. That means, a well-planted tank, with dim lighting, tannin-stained water, and a very well-fitting lid. That should bring out the most natural behaviour, and best colouring.