Twitter didn't block that post for that reason. Also it wasn't on Trump's account, it was the NY Post which is, as you say, a tabloid.

As you can see here, he posted about it and there is no warning label:

https://twitter.com/realdonaldtrump/status/13178254844958801...

Didn't it just turn out not to be true?

No, the story simply had very little evidence and so had no definitive claims, but for now there is no evidence that those email where planted by foreign agents nor that the content of the laptop was falsified (for example while the Biden campaign dismissed the story they never claimed the emails to be false).

Moreover a witness that was part of that email thread confirmed the conclusion reached in the original story.

That was blocked for privacy violations. And Twitter allowed discussion of the story, just not hotlinks to the story itself which was publishing stolen private content.