I believe there is confusion about these terms. Strictly speaking, the ESC is PWM controlled. Although the signal usually gets there from a transmitter and receiver signal that is PPM encoded.
It seems to me, that many sources will have you believe that PPM is just a multiplexed PWM like this:
But in reality it is actually more like this:
Note in the final diagram that the actual PPM signal does not vary significantly in pulse width, only it's position is significant. Hence the term Pulse Position Modulation. From those positions, a controller will reconstruct a PWM signal that resembles what appears in the "Partial PPM Pulse" diagram (and is unfortunately labelled as PPM). That "PPM" signal (actually a multiplexed PWM signal) now has widths that are significant.
Rather unfortunately, neither that true PPM signal nor the so called PPM (actually multiplexed PWM) signal will make your ESC work correctly. It also has to be demultiplexed so that the information for only a single channel is fed to the ESC (or servo). This is in fact a PWM signal again.
It might be because the PWM has to have specific dimensions to work on the ESC that they don't want to use the term PWM, in other words its not just any generic PWM signal. Specifically the pulse width usually needs to be 1500µs long (+/- ~500µs) and occurs 20000µs apart (hence the 50Hz frequency), but there are alternatives to that scheme. Compare this to a PWM signal that can be used to control the brightness of an LED, for example, which can have any frequency and the widths can vary between 0% and 100% of the duty cycle. These are vastly different requirements from the PWM signal required to operate the ESC.
As far as I am aware, most ESCs used in RCs will have this type of control schema.
So in conclusion:
PPM when used to refer to ESCs is meant to be a specific kind of PWM. Any ESC that says it is PPM or PWM compatible is referring to the same thing. An alternative would be a serial motor controller, but I have never heard of one that was referred to as an ESC. Those are simply "motor controllers". For example, these ones. Also not that some of the controllers on that page are also PWM controlled, but it is not the same kind of PWM -- more akin to the LED brightness control I mentioned above.