Did she start Twitch pre or post Call of Duty, though, and get the gig as result of being a Twitch streamer? When I look at her Youtube channel, oldest video is 6 months, so it strikes me more as capitalize on the popularity vs. why she got the job.
Guy who played Connor in Detroit: Becoming Human started streaming a lot when the game became popular, but he didn't get the job because of Twitch streaming, he just tried to capitalize on the popularity.
Honestly it is smart business sense, because game popularity can be fleeting and it increases your visibility to increase odds of later success.