There’s a very specific legal definition of what a margin account is.

Robinhood accounts are Regulation T (aka Reg T) margin accounts by default. They have limits, yes, but they are very much margin accounts. That’s how they make their UX work.

The point is that typical user activity will be performed on margin, transparently. The flurry of synchronized activity caused a larger than normal draw on their margin, which further strained their working capital.