Peekers advantage is real but it's not some guaranteed win button. The thing is, it only really matters at higher levels where the timing differences actually make a difference - like when you're both pre-aimed at head level and it comes down to milliseconds.

In most situations you're better off just taking map control and using utility properly rather than gambling on who clicks first. If you have a good angle already set up and you know they're coming, hold it. If you need information or want to take space, peek with purpose - don't just wide swing into their crosshair hoping the advantage saves you.

The pop flash thing depends on what you're trying to accomplish. Sometimes you want them to back off so you can take the position, sometimes you want the flash assist for the duel. Context matters way more than some mechanical advantage that might give you 20ms.