I don't know precisely what the article means. The NYSE Retail Liquidity Program, which still exists, describes two categories of participants: member organizations (MOs) and liquidity providers (LPs). MOs have retail orders, which are defined as originating with an actual person, ie not a computer. LPs provide liquidity to those orders, ie they take the other side of the trade.

I believe Knight would have been interested as an LP. It is not inconceivable that in some circumstances Knight would have been able to submit retail flow as an MO, but 100% of that flow would have been routed to it from brokerages holding actual retail accounts.

Knight was a trading firm, not a hedge fund, and certainly not an institution which held outside money in retail accounts. But consider an entity which does have retail accounts and also has propriety trading for its own account. Suggesting that the former would become inaccessible if the latter lost lots of capital in bad trading is absurd, would mean that retail-customer and proprietary monies were mingled, and would require the violation of untold numbers of regulations. This did not happen with Knight and indeed has never, ever happened.

You're misreading because the sentence is unclear possibly because the author doesn't know much about market structure.

The NYSE program was to provide improved pricing to retail investors through retail brokers. The people who provide the pricing to the retail brokers were market makers like Knight, Jump etc. You didn't have a brokerage account with Knight, your broker would trade in the market and Knight would take the other side.