What business incentive would they have to be a black car app, if they can make more money by expanding into UberX? Not to mention, if they had remained a black car app, they might have gone out of business.

Uber is a business, not a charity. They exist to maximize profits. If you understand that point, then you see why your point is irrelevant.

Also, what the heck does it mean to "abuse growth"?

If I had to guess, abusing growth means taking advantage of the fact that a lot of investors use growth as a core metric in their decision-making process. Thus, Uber optimising for growth over any other "more reasonable" option, just to obtain funding.