Is the intuitdatalitigation.com legitimate?

It appears to be.  It also looks like a classic class-action scam where the lawyers split millions and the class of injured parties get worthless "benefits" (since there are many ways to get free credit monitoring without being part of the class, getting it seems to be an essentially worthless benefit.)

Note that the letter indicates that you are a member of the class and eligible for the free credit monitoring only if the IRS determined that your identity was used with TurboTax by someone other than you to file a fraudulent tax return in your name.