A down-stepping DC-DC converter is not an inverter?
Nope. But the pieces of the implementations are similar enough in function that it's close.
A typical DC/DC down converter involves two switches, an inductor, and both input and output filter capacitors, plus control circuitry to sense the output voltage and time the switches. (There may also be a VERY small resistor in series with the inductor to sample the output current if current regulation is necessary, but it's omitted for high efficiency if that's not an issue.) One end of the inductor is hooked to the output cap, the other through the switches to the input cap and to ground.
The pull-up switch is always active (typically a transistor). The control circuitry turns it on and the current in the inductor ramps up, charging the output capacitor at an increasing rate. After a while the pull-up switch is turned off and the pull down switch is turned on. The current through the inductor ramps down, but before it goes through a stop and reverses the pull-up switch is turned back on and the pull-down turned off. The pull-down switch may be a diode, which switches on as needed automatically, but for high efficiency it's usually another transistor, because it has a lower voltage drop and thus is more efficient.
The control circuitry varies the percent of pull-up versus pull-down time to keep the average output voltage at the desired level. The frequency may be controlled or may be allowed to vary somewhat.
So the waveform in the inductor is a sawtooth, and the current never reverses. An "inverter" by definition,