Does Impedance Matching Imply any Practical RF Transmitter Must Waste >=50% of Energy?
If your power source is a zero ohm output voltage source, followed by a 50 ohm resistor, then yes, what you think is correct. 

However, practical RF amplifiers (at least ones designed to be efficient) are never built like that. They tend to have a low impedance common emitter or source stage followed by reactive impedance matching, all designed to operate into 50 ohms.

Interestingly, if you buy a general purpose signal generator, the output is usually built as a voltage source followed with a real 50 ohm resistor, as efficiency is not an issue, and having an accurately defined output impedance over a very wide frequency range is the main design goal.