This is true ... except that it is simplistically naive way of looking at things, because this is just one form (out of many) of anti-competitive practices. It is essentially high-school level elementary basics of anti-trust. In actual reality there is quite a bit more to it than that.

For instance: Monopolies often don't actually limit supply. You only make it so customers can't choose an alternative and set prices accordingly (that is higher than they would have been if there were real alternatives). Big-tech companies do this all the time. Collusion is also not required, but only one form (today virtually unheard of or very rare) of how it may happen. For instance: big-tech companies often don't actually encroach on core parts of the business of other big-tech companies. Google, Microsoft and Apple or Uber are all totally different business with little competitive overlap. They are not doing this because of outright collusion. It's live and let live. Why compete with them when they are leaving us alone in our corner? Also: trying to compete is expensive (for them), it's risky and may hurt them in other ways. This is one of the dirty little secrets: Established companies don't (really) want to compete with other big companies. They all just want to protect what's their and keep it that way. If you don't believe me have a look at the (publicly available) emails from execs that are public record. Anti-competitive thinking through and through.

In the classical economic sense, Lyft/Uber should be competing to drive prices down to razor thin margins for the facilitator service. Is that happening? Or are they pocketing fat margins?

And it wasn't much of a cartel in NYC before, anyways. Most subways stops in Brooklyn had a black car nearby if you knew how to look for them.