You'd be hard pressed to find any sort of consensus that Tatum is a top 5 player in the NBA and definitely no consensus that he is top 2 in the NBA. Tatum, even if he is playing at a borderline MVP level, is certainly behind Giannis, Jokic, Doncic, and Curry, and nearly certainly behind Embiid and Durant, but safely in the next tier and top 10.
Tatum was First Team All-NBA last season. Basketball Reference has him as their #4 ranked MVP candidate for this year to date. The Celtics made it to the Finals last year and have the best record so far this year (which generally gets consideration in discussing top players). His counting stats and efficiency stats continue to improve each year. Apparently, some people already view him as a Top 5 player. Last I checked, he was 1st in Real Plus Minus, 4th in Plus Minus, 4th in Win Shares, and 5th in VORP. Sure, he might be behind Curry, Embiid, Giannis, and KD in some categories, but Tatum suits up for more games . . . isn't availability a component of ability? Not sure what constitutes being one of the top players these days? Team wins? Counting stats? Rate stats? Efficiency stats? Post season results? Supporting cast?
I brought up that last part because the Celtics also have Jaylen Brown. In the games when Brown hasn't played, Boston is 5-0 with a +16.6 scoring differential with Tatum averaging 36.4 ppg. We can debate if Brown helps or hinders Tatum being an upper tier player or not. Brown certainly helps the C's win more games, but he averages a shade over 20 shots a game. The other marquee players in the league typically have a guy shooting 15 times a game. Tatum isn't hurt for shots each night, but a few extra here and there might help his counting stats.
All that being said, I still am not convinced that Tatum is a guy that Boston can rely on to close out games, get a big bucket in crunch time, and make a handful of good decisions down the stretch to walk out of the arena with a W instead of an L in a playoff game. We've seen guys like KD, Luka, Curry, etc. make those gotta have it shots when it really matters. Tatum came up huge in the Game 6 elimination game in MIL . . . but I don't remember other mega important games that Tatum was the go to hero. Brown has generally been the one that Boston has had to score in bunches in the 4th quarter.