Another problem that is less obvious is that GFCI receptacles and breakers are not listed for back feed. Plugging this into an outlet on the patio would mean using a GFCI circuit. If the GFCI tripped while the PV was back feeding it would burn out the electronics that make it work. The outlet or breaker could be reset but it will not trip on a low amperage ground fault.
Grid tie static converters are "line commutated" and vast majority of them aren't even capable of black starting and plainly lacking the timing circuit to even do so.
Feeding power into a circuit such that it results in the production of exported power without working with the utility is a poor idea though. It will sometimes hurt you in your wallet. For example 10kWh of imported energy and 2kWh of exported energy means 12kWh passing through the "highway" and smart meter software can totalize it in a way that results in you getting billed for 12kWh of use.
If you have a very old school treadmill with mechanical transmission and no electronics, then you could do the same "back feeding" by putting it into generating mode by driving a motorcycle on the belt such that the motor turns into an alternator.