That's an extremely poor article. Anyone can throw a hot take on SeekingAlpha; that doesn't mean their take is worth anything.
1. I don't know why the title mentions "millenial" customers in particular, because every brokerage does this across all demographics,
2. This activity is not "selling out" customers. If you believe that customers are "suckers" because their order flow is sold to high frequency traders, you have a very fundamental misconception about high frequency trading and its role in market making,
3. There is no codified definition of, or law protecting, "financial privacy" in the sense of order flow. This data isn't connected to you as an individual, just as you can't see which individuals or companies are placing bids and asks in the order book just because you can see the amounts and prices. All trades in the market are publicly reported regardless of whether or not your order flow is sold.
It never ceases to amaze me how the term "high frequency trading" can compel people to pontificate about things they clearly don't understand. You'd think we'd have collectively moved on from the Flash Boys misconceptions by now. Yet here we are, with an article talking about high frequency traders as some kind of financial boogeyman in 2018.