If seeking a platform makes you a mouthpiece, then ISIS is just a mouthpiece for Twitter.

ISIS aren't employed by Twitter. It's completely different.

Amy Goodman hosts a show on NPR, a U.S. government news station. She is literally a mouthpiece for Trump and the U.S. government.

RT is owned and primarily funded by the Russian government.

NPR receives around 2% of its funding from the US government.

Also, as per Wikipedia:

"Democracy Now Productions, the independent nonprofit organization which produces Democracy Now!, is funded entirely through contributions from listeners, viewers, and foundations and does not accept advertisers, corporate underwriting or government funding."

(https://en.wikipedia.org/wiki/Democracy_Now!)

You're right. I should give you the benefit of the doubt and make the real argument, which is that if a government provides a platform for you to make commentary, it does not necessarily mean you are a shill for that government; rather it means that the things you want to say, a government is supportive of.