The article is completely wrong. Robinhood did not disable buys only, they disabled any trades that took a net new position. Existing options holders who needed to buy or sell were allowed to, and regular retail traders could also buy or sell on the other side of those transactions, because they went to closing existing open positions.

The reason you couldn’t buy but you could sell is just the simple mechanics of a short squeeze. All the net open positions were short sellers needing to buy (e.g. needing retail investors to sell) and there was just near zero volume of call options holders looking to sell (e.g. needing retail investors to buy) to close an existing position.

I feel pretty upset to see the article going into all these details of capitalization - that 100% was not the issue and at no point did Robinhood “disable buys but not sells.” They disabled creating net new positions which just happened to have a side-effect of disabling buys as a matter of the particular trading volume of various option types. If you were a call option holder, you could sell (and therefore the other party could buy).