> Later that night, at 3:26 a.m., the company sent an automated email demanding Alex take "immediate action," requesting a payment of more than $170,000 in just a few days.
They did not say, "one leg of your trade is now $170k in the red". After all, apparently that leg was $700k in the red. No, according to the article (which may be misleading, but I'm just trying to tl;dr it not run an independent investigation), Robinhood claimed that the customer needed to send them $170k immediately with unspecified consequences for failing to do so. If the debt could in fact be covered by other positions in the account (which is what the article says the customer's lawyers say), then Robinhood's claim is a lie. The customer did not need to transfer $170k into the account, there were other actions he could take.
You say he should have known that he had other avenues. Indeed, I acknowledge that explicitly in my original comment when I point out that the customer knew that what Robinhood had told him could not be true. But because of the knowledge imbalance he apparently came to doubt his own knowledge and believe the lie. That is why such a lie is particularly heinous, just as on the other case I mentioned.