That load sharing circuit works when the input power is 5V USB power. But he's wrong about using it with a solar panel. In low light, the panel will produce some small amout of current, but the panel voltage will rise to the level of the battery voltage. That will raise the gate voltage and turn off the mosfet when the battery is needed to provide most of the load current. As a result, the battery current will flow through the mosfet's body diode, with a resulting 0.7V drop. So at the very worst time, when the battery has been working all night and is mostly discharged, when the light comes up even a little bit, the voltage will drop by 0.7V, and that may shut down your circuit.
I have a Github repo devoted entirely to this problem, and if you have nothing else to do you are welcome to read it.
https://github.com/gbhug5a/Solar-Power-Load-Sharing
The load sharing circuit for use with a solar panel is more complicated, and includes an opamp. But are you sure you need load sharing? If you can put your circuit to sleep periodically, charging termination may still take place properly, and a load sharing circuit wouldn't be absolutely necessary. Can you tell us more about your project?
Edit: I should have said that another option is to use his load sharing circuit, but add a schottky diode across the mosfet. You still have a voltage drop in low light, but a smaller one - more like 0.3V. But that drop will rise as load current increases, so that's not a great solution.
