Although they've left the tweet up, the article it links to says that this is incorrect: "Correction: An earlier version of this article stated that 56 percent of Robinhood users hold GME stock. This is incorrect, based on a misreading of a statistic on Robinhood. Motherboard regrets the error."

I can say with 99% certainty that they aren't just counting invidual $GME holders, but every one of Robinhood users that holds an ETF (like total US Market) that contains $GME...

So this is really such a bad faith statement from Motherboard, because all of the ETF holders (probably 90% of the "half") can still sell their ETF's and thus their indirect $GME shares...

I think GME is one of the freebie stocks that Robinhood gives out as a referral bonus, so many of those users might just have had one stock incidentally.

Can this really be true? It sounds so implausible that I'm not sure if I can take their tweeted word for it.

From what I understand Robinhood's actions today only impacted derivatives of GME, not trades involving the actual underlying equity (which, I imagine, is what most people actually own).