And all of that works because the temperatures on Earth are way less than the surface of the Sun. One could (in principle) build a device that heats up until it’s the same temperature, about 6000 K. But you could never build a “one-way valve” that keeps absorbing energy above that.
The plants (and other stuff) in a greenhouse absorb visible light at a high temperature (short wavelength) and emit it as infrared at room temperature (long wavelength). So as you noted, the light can be let in but blocked on the way out, since it’s a different wavelength. But if the stuff got hot enough, it would be absorbing and emitting at the same temperature, and there’s no way to block that in just one direction.