I'd say he's at their level for different reasons.
Flair is primarily known because of his life of being in the wrestling business. He was a transcendental personality because his numerous legendary matches and promo work managed to capture the imaginations of even surface-level fans of wrestling at its peak in the 80's and early 90's.
Hogan, Austin, and Rock were the first to really cross-over and be all-around pop culture stars, with Rock being obviously the most successful of the three. Hogan's ability to reinvent himself in the 90's as the bad guy in the NWO was inspired at the time and while it ultimately in some respects helped lead to the downfall of WCW, his impact was such that you couldn't talk about pop culture in those years without bringing up Hogan and his effect on pop culture. Austin got to that level more similarly to Flair, with some minor cross-promotion between his television shows, podcast, and movie appearances.
Cena to his credit helped build up a lot of his cred through his work with Make-A-Wish. As much as myself and others despised the guy for being essentially being too much of a babyface (seemingly always coming out on top in the end, always being one-step ahead, burying competition so hard that they could never recover in some cases) his appeal to children during his full-time run in WWE was unmistakable. Post-full-time WWE, he's been an advocate for a lot of good causes and has transitioned nice to several roles in Hollywood, commercials, and the like. He's super marketable and generally seems like a nice, handsome, impossibly jacked dude.
You could likely make the argument that Rock is far above any of the guys mentioned, but generally speaking as people who have had a great effect on pop culture, Cena's place I don't think can be denied at this point, and he's no where near done.
