That's quite literally what gamifying is: providing the kind of visual rewards/stimulus, combined with an extremely low friction experience, that's specifically designed to work in concert to encourage engagement.
In the case of a slot machine, that means putting coins in the machine and pulling the lever.
In the case of Robinhood, that means executing trades.
Maybe you don't believe those features of Robinhood actually encourage trading, but frankly, the psychology around RH's design is pretty well understood both in the technology industry and the gambling industry. Hell, there's an entire booked (Hooked!) written about it.
> Do they need to make their UI flow less smooth, or give some big warning that people are dealing with real money?
Yes. Absolutely. Why would that be a problem?
Ostensibly RH's mission is to provide people with free access to the markets to "regular folks" (read: inexperienced traders).
That's not the same thing as encouraging day trading.
I don't see why features that discourage excessive trading (which is unquestionably an anti-pattern for a typical retail trader, and therefore is not in the best interest of the folks RH is supposedly trying to empower) would be a bad thing...
... except, of course, RH's revenue is specifically derived from high trading volume, so it's not in their financial interest.