The phrase "you've gotta spend money to make money" is most commonly used to justify an investment that won't make money, amirite?

Yep, pretty much

Yes and no. It's kind of a truism, you must invest to reap returns, but it's probably something that's routinely brought up to sell questionable investments too.

As a mechanic, it's literally true that I have to spend money to make money, every job that rolls through my shop requires I spend money on it. I get more back in the long run, but if I didn't have any funds to invest in jobs I'd be dead in the water no matter how much work came in.