Stablecoins are securities, in my humble opinion, and should be regulated as such. I believe that is the point the author was making as well.
"Securities are fungible and tradable financial instruments used to raise capital in public and private markets"
It's a financial instrument, tradeable, and used to raise capital in public markets.
How is a stable coin not any of these?
A user might decide to use a stablecoin as a fast and low-fee transfer mechanism for remittances and donations, or to manage a multisignatory account, or to hedge against the falling price of a crypto asset, or for some application of escrow in a smart contract.