No, Galaxy of Heroes does not make comments on what is or is not canon. Galaxy of Heroes units are Warhammer 40k minis, and can be drawn from whatever, regardless of its canon status.
The word "Darth" has the literal, canon, in-universe translation of "Dark Lord." That is what that word means. Dark Lord Maul is both a Sith and a Lord. Dark Lord Maul is a Sith Lord.
And your opinions on the sequel trilogy really don't matter, so drop it before this turns into another bickering about The Last Jedi thread. You don't like the movies. I do like the movies. Whatever.
That said? The Force Awakens, Rogue One, The Last Jedi, and Solo happened. They are real. They are canon. They were issued by the rightful owners of the IP.