It's not subjective. The Oxford English dictionary definition is "The best or most important thing owned or produced by a particular organization." https://www.lexico.com/definition/flagshipthe term 'flagship' may be subjective but i think it is safe to say in the context of gpu's at least the vast majority of people will and have connected that term with the fastest and you saying it's not will not change that fact..
In this case it clearly refers to the overall best and most important in product in the Ampere lineup, which is the 3080. The 3090 is of no significance or importance other than a BS 8k marketing exercise.