I believe you are wrong. The NBA changed this rule a year or so ago and even if they win the Atlantic, the Celtics can finish the fourth seed.
Did you read the article? ESPN has been reporting that Atlanta would hold the tiebreaker, and an NBA source reached out and told them they were wrong.
Here's the full article:
Recently, we've been noting that the Celtics must finish with a better record than the Atlanta Hawks in order to secure that third seed because our interpretation of the NBA playoff rules suggested that the first tie-breaker between two of the top four seeds reverted to head-to-head record (with the Hawks having swept Boston in four regular-season meetings).
Turns out it's even simpler. According to the NBA, a division leader automatically trumps a second-place finisher in another division. So, assuming the Orlando Magic win the Southeast Division (they're 4.5 games up on Atlanta), it's actually the Hawks that will need to finish ahead of Boston in order to net that third seed.
We bet Atlanta's not thrilled with that tie-breaking procedure considering the strength of the respective divisions. Especially since, under a tie scenario, the Celtics would boast homecourt advantage in the Eastern Conference Finals should the teams meet there.
Thus, if that's false, it's the NBA source that is wrong, not Evantime.
The Celtics can have the fourth seed, but apparently they automatically win a tie-breaker if their record is the same as a non-division winner (Atlanta).