This is a really good article.
The net interest thing has a parallel in the HF world. CTAs, a type of hedge fund, used to do substantially the same thing. The margin requirement for holding a load of futures is quite low, but the customers of the fund have put cash into the fund. So back when interest rates were higher, a fair bit of a CTA's returns would be interest on the cash.
He's right about Robinhood. From the HFT side, the problem is if you're in the "real" institutional market, someone will run you over occasionally. With retail, that never happens, and the spread you can offer is thus tighter. There's no front running to it, just the ease of non toxic flow. So RH is trying to put forward zero comms as a way to get some deposits.
He's also right about options. It's tempting to think that you can just sell some options to collect premium, or do a combination of some sort (iron condors, butterflies, etc) to save yourself from wasting money. But keep in mind the costs when you're doing this, and the fact that you are simply paying up to squish some risk into an unfamiliar shape. It also has a gambling-like tendency to create occasional wins to keep you interested.
As for investment advice for retail people, if you are a coder there doesn't seem to be any reason to use anyone other than IB. They are the cheapest, and if you can code you can use their APIs to trade whatever it is you want. I'm not saying anyone can make a great trading strategy, but you can certainly manage some passive investments with barely any fees. Or just take advantage of RH and do it free I suppose.