I don't know if the show one, show all rule applies. I haven't read the rule lately, but I've always seen it work as a rule that only applies on demand. I think maybe once or twice in a hundred of those situations does the dealer retrieve the cards and show them to everybody, unless somebody actually asks to see them.
But personally, I think intentionally flashing his cards at showdown, especially if the dealer sees them, functions as constructively tabling his cards. He showed his cards - I don't think the fact that not everybody saw them, or that he didn't carefully place them face up in front of him matters.
Flush wins, I say.