At the end of the day, other teams knew it was expiring and probably drove for hard bargins I wager
It depends on what the motivation of the other team was. With the Celtics' TPE expiring, those teams also lost one of the few avenues of completing a pure salary dump. Now teams are left with just the Spurs and the Pacers, I think? Those teams are unlikely to give positive assets to acquire a veteran player, meaning the selling teams will now need to attach an asset if they're looking for salary relief.
My guess is that the Celtics looked at what was available, and decided it just wasn't worth the monetary cost. If you assume the Celtics cleared around $100 million in profits last year (perhaps a tick more), and would have made around $100 million or so this year if they'd kept to the luxury tax, then the Celtics current payroll makes sense.
We're about $20 million over the tax right now, and looking at $44,518,789 in payments. So, assume we're $65 million over the standard budget. $100 million in profits turns into around $35 million now.
Then, we've got three roster spots. Assume those come in at $5 million combined (which is on the low end). The luxury tax hit on that $5 million is $18.75 million. So, total cost on filling those spots with cheap contracts (like Matt Ryan cheap) is $23.75 million. So, $35 million in profits is around $11 million now.
Any salary beyond that is multiplied by 4.25 for the first $5 million; 4.75 for the next $5 million, etc.
So, operating income (profits) are relatively stable, Wyc can add about $7 million total to fill the three roster spots. That would put total payments at around $100 million over the tax line.
Now, maybe the team got a revenue bump due to the Finals run, so the numbers can be adjusted slightly upward. But, I don't think many ownership groups will operate at a loss unless it's to significantly increase their chance at winning.
(Reminder: these are rough estimates.)