It's poor advice for someone who already has a STEM degree and wants to build something useful and profitable. If you already know how these things work, your time is better spent on the "edge of the circle": http://matt.might.net/articles/phd-school-in-pictures/ which applies to businesses and startups as well.
If you're in the latter group -- you've already got the skills to build real shit. Don't waste your time on homework problems. Find a problem you have and build a solution for it. Don't listen to people who tell you to work on homework problems that have already been solved; it's a complete waste of your time if you already know the fundamentals.
As for stock trading bots -- if you don't have a mathematics degree or equivalent (e.g. having incredible math skills), don't even bother. You won't be profitable, and you will learn nothing useful in the process, because you will approach the problem as a naive CS student would. Smarter people than you have made trading bots and have failed miserably. Without having an extremely strong foundation in mathematics, your trading bot will amount to nothing more than a futile exercise in gluing APIs together.