What, using investor money to offer below-cost fares, is not a form of subsidy?

No, it's a form of business. Big companies often drive the price down, so that they can enter a market dominated by entrenched players. They are able to do this by using their profits from somewhere else. In this case, the money was not directly earned by Uber, but the investors funding Uber.

Uber is using investor money to offer rides below cost - i.e. someone else is footing the bill, you are taking a ride subsidised by Uber's investors.

It's just semantics and I'm not sure why you think it's important.