I don't think it was an issue to not call the timeout. You have Tatum and Brown on the floor. Let's be honest the final play was going to be one of those 2 goes iso, whether they used the TO or not..
The first issue was that Tatum started his move 45 feet from the hoop with 5 seconds left, even though there was no ball pressure the entire way up the court and he got the ball with around 7. Tatum got Maxey on him on the switch. Even got past him and turned the corner. Everything went right, except that instead of hitting the open guy on the double (White) or taking the shot once he was in the lane (Embiid was still rotating over), he took that extra dribble, found himself 1v3 under the hoop, and whipped a 28 foot pass with 1 second left
I think that one is 90% on Tatum, and 10% on Smart for not getting Tatum the ball until 7s left