> I assume it is a commentary on Brexit.
I feel more that it is a commentary on "blind nationalism" of which Brexit is one example, but not the only one, or the most recent. Brexit may be "over" now, but the mindset is still very much with us in the UK and elsewhere. In other words, any successful art relates to more than one specific situation, and allows more than one reading.
It probably is partly about Brexit. (I voted Remain by the way!!!) But he has made the blank flag so general that it could be interpreted to be so many other things like a NATO flag, an Esperanto flag, the flag of Essex or an autism pride flag.