I'm not sure if we can be 100% sure of that until we ever see the male version of Garchomp... cause for all we know, they just randomly chose to use that version and didn't care about the gender thing
With May's Venusaur I'd say it was confirmation of it being female because they never drew Venusaur like that before, so they clearly wanted to confirm it as a female, but with Garchomp we didn't get to see another one to be sure they intentually made Cynthia's female...
I don't know, I think it's hard to say it's 100% female just cause of that...