No. Terminator: Dark Fate is not a sequel to Genisys. Emilia Clarke, who featured in Terminator Genisys as Sarah Conor, refused in her speech in April 2016 to return for any sequel. So, when paramount was set to release the 2017 schedule series, they made it known in January 2016 that the sequel has been removed. In the Terminator: Dark Fate, Linda Hamilton features again in her role as Sarah Conor. So as to ignore the continuation of the previous series, and reboot the franchise, Terminator: Dark Fate has refused to follow sequel with the Terminator: Genisys, Terminator: Salvation and Terminator 3.
James Cameron, who happens to be one of the writers of the movie, considers the Terminator: Dark Fate as a sequel to The Terminator, which was released in 1984 and Terminator 2: The Judgement Day. But the other series of the terminator movie, such as the Terminator: Salvation (2009), Terminator 3: Rise of the machines (2003), The Sarah Conor Chronicles (2008-2009), and The Terminator Genisys (2015) are actually of the different storyline.