The post-up came back and nobody announced it

It returned as a passing hub rather than a scoring move, which is why the people who declared it dead never noticed it breathing.

Post-ups were declared extinct roughly a decade ago on solid evidence. Points per possession on a post-up were poor, the shots generated were long twos and contested hooks, and the whole action stalled everything else on the floor while four players watched. Teams stopped running them, and offences got better. Case closed.

Except go and watch a good halfcourt team now, particularly late in a close game, and count how many possessions start with somebody catching on the block or the low elbow with his back turned. It's more than you'd expect.

The difference is what the catch is for. The old post-up was a scoring attempt with a pass as the fallback. The modern one is a passing hub with a score as the fallback, and that inversion changes the entire economics. Once a defence sends a hard double from the top, four offensive players are guarded by three, and the read is not hard: skip to the weak-side wing, who attacks a rotating closeout, and now you have a scrambling defence with a shooter relocating behind it. That possession doesn't get charted as a post-up. It gets charted as a spot-up three, which is exactly why the numbers said the post-up was dead.

The personnel changed too. A post-up used to mean a slow centre grinding for eight seconds. Now it's often a strong wing sealing a switched guard for two seconds, catching on the move, and either turning over the shoulder immediately or kicking out before the help even gets there. Two seconds, not eight. The clock cost that made the old action so damaging is mostly gone.

There's one more thing that makes it valuable and it's the reason it shows up in playoffs specifically. Post-ups are the best answer to switching. If your opponent switches everything to kill your pick-and-roll, they have created a mismatch somewhere by definition, and there is exactly one action in basketball that punishes a size mismatch without needing a screen, a handoff, or any co-operation from a defence that has decided to guard you one-on-one.

So the honest summary is that the analytics were right about the action they measured and wrong about the action that replaced it. Most tactical arguments in this sport end that way. The thing everybody buried came back wearing different clothes, and the tracking categories, which never update as fast as coaches do, kept filing it under something else entirely.