Most inverter generators are a "good enough" sine wave.
I think you're missing the point of inverter generators. The root problem is that regular gas generators cannot produce a "clean" sine wave because they change speeds slightly during operation. When the speed changes, the wave changes slightly. This isn't a problem for regular electrical devices like incandescent lights, power tools, "basic" refrigerators, etc. They merely need the flow, not so much the sine wave.
Anything in the "consumer electronics" category, however, is not so lucky. Most of these devices rely on the sine wave for timing, and normal generators won't cut it here. I remember trying to watch TV on a conventional generator after a hurricane a couple of decades ago. The sound was fine, but the tube TV couldn't fill the whole screen. The TV ran fine once power was restored.
Inverters fix the issue by inverting DC to AC. As the output is consistent, consumer electronics will run relatively well on them. That's the main reason you buy these types of generators. If consumer electronics won't run on them, they're overpriced regular generators.
"Pure Sine Wave" seems to be a marketing term
Having looked at several inverter generators, "pure" and "clean" are only sparsely used. This Champion generator says
Clean Power for Sensitive Electronics. Includes two 120V 20A household outlets with clean electricity (less than 3% THD)
This pricey Briggs and Stratton generator doesn't mention sine waves, clean or THD. It merely says
The inverter generator technology supplies consistent power for essential home appliances, including sensitive electronics.
And finally, your Ryobi RYi2300BTA with it's "pure sine wave"
THD < 5%
The real spec to pay attention to is Total Harmonic Distortion
Total Harmonic Distortion is a heavy concept, but it is a real metric
Total Harmonic Distortion (THD) is the stated measurement related to the quality of electricity.The amount of THD will influence how your equipment may respond or perform.
When power quality is discussed, you will hear terms like “clean electricity” and sine wave power. What these refer to is the lower harmonic distortion of the electrical AC wave and the ability to take square wave generated electrical power and smooth the edges to produce a phase that is smoother and less square.
“Clean electricity” is considered good with a THD of less than 6% and often stated or promoted at 5% or less.
Sadly, most marketing ignores this metric. They all likely refer to a THD of less than 5%.