There is no need for a boost converter to convert 5V to 10V. The Meanwell has a built-in 100μA current source pulling the input up to 10V. All that's needed is a transistor to short the PWM input together. If nothing else in the ESP8266 circuit is connected to the Meanwell driver, then there will be no need for an opto-coupler. And no, if you do use an opto-coupler, the transistor side should go to the PWM input, with the ESP8266 circuit's PWM output going to the photodiode.
To expand further, the Meanwell's input looks like this:
The idea is it will work with a 0 to 10V, a resistor, or PWM input. If a resistor is connected to the input, the 100μA source will pass a current through it up to a maximum voltage of 10V. For example, a 10k resistor will drop 1V across it, with 100μA through it. The low pass filter removes the AC component, so if you apply a 50% duty cycle 1kHz 10V square-wave, it will be smoothed to 5V.
Connecting a transistor between the dimmer inputs will short them together, when it's on and the voltage will increase to 10V, when it's off, thus the logic will be reversed.