I think Tatum himself started the speculation that he would continue to grow. I believe he claimed in high school that doctors said that he would grow to 6'10 maybe 6'11. Then there was a rumor that the Celtics thought this might happen too and supposedly his growth plates had still not closed.
It seems that Tatum has definitely grown at least an inch or so since last year, lending credence to those speculated claims. No doubt he is a legit 6'9 (nba standards: on court, in shoes) or perhaps even 6'10 now.
The difference between 6'9 and 6'11 was huge for guys like Giannis and Durant, in my opinion. It made them different players. This is why I think people are so interested in Tatum's height and mythical growth potential.