He has not kneeled.
He hasn't even said he will.
He has said he would not be surprised if some baseball players do, and that he's in the process of thinking about what, if anything, he wants to do or not do, and that he feels bad that baseball in general didn't do much to support the player who kneeled during the anthem, Bruce Maxwell, who was not very surprisingly phased out of the game.
You're certainly right that not everyone has to kneel.
Every person is free to do whatever they like during the national anthem, just like every person is free to listen to or ignore the facts that most if not all athletes who have taken a knee during the national anthem have stressed that their intent is not to dishonor the flag or the military, but to promote a conversation about systemic racism in American and some of the ways it presents itself.
Every person is free to determine their feelings about the flag and what it represents. Some, for example, feel that the flag code says pretty specifically that the flag should not be used as a prop on shirts and hats and other clothing items. Some think wearing those shirts and hats are patriotic.
To each their own, but the great thing about America is that we get to decide.
As for Flaherty's so-called grievances, he's a black man in America.
Being a professional baseball player doesn't mean he's not immune from the experiences that are unique to black men in America.