It makes sense to consider a criminal defendant's character for truthfulness differently than that of other witnesses, because they have something at stake (liberty) which nobody else does. And remember, it's natural to consider the defendant inculpated, at least slightly, by the decision not to testify. And if they do testify, everyone thinks they have a strong motive to lie. So damned-if-you-do-damned-if-you-don't, and therefore we give them this bit of leeway. And that principle is old: it's not newly codified.
And note that the defendant's character for truthfulness is put in play not just by the simple act of testifying, but by offering evidence about character for truthfulness (i.e., by bringing in character witnesses). By testifying, the truthfulness of the testimony itself is at issue, of course, but not the character/propensity to be truthful. So merely testifying doesn't allow the prosecution to call witnesses to testify about my character for untruthfulness.
So "did he lie on in his testimony" is different from "does he lie in general." Sometimes (perjury, hinering/lying to an officer), character for truthfulness may be relevant whether the defendant testifies or not: it's directly related to the elements of the charge.
But this is also a useful case, because it explains the risk that one takes by calling a character witness. If you put on a character witness, "have you heard" and "did you know" questions are permissible on cross: it's a way to get otherwise wholly inadmissible evidence into the record. Not to prove that the defendant is untruthful, but to test whether the character witness really does know the reputation of the person on whose behalf the testimony is offered, and has an opinion worth listening to. So you can ask about juvenile misdemeanors for example. (Would it change your opinion of his truthfulness if you learned that he had shoplifted as a 15-year-old?)
So this isn't in the rules of evidence, but it is still the rule. It's sort of a 608(b) or 405(b) thing: the character trait has been made an essential element of a claim or defense, so it can be impeached with instances of specific conduct.
The jury isn't just using this for identity and intent, of course, but that's why it's being offered. Even though there's a permissible purpose, it's likely that the jury would use it for the impermissible purpose, in spite of some limiting instruction.
Posner says the idea that evidence is "inextricably intertwined" is just hogwash. It's better just to look for a permissible purpose under 404(b). Or even a purpose not listed there. Just don't offer it for the FORBIDDEN purpose. So the facts here are unremarkable, but Posner requires us to be rigorous (at least in 7th Cir.) about why we're admitting evidence about other crimes which would otherwise be impermissible character evidence.
To show that a witness has a reason to lie now (motive): it's payback for some earlier things. It doesn't even have to be a criminal case. That's a kind of impeachment, of course; but it's a way of using prior bad acts to get there.
In the 80s, when rape shield statutes started to become commonplace, there was outrage that defense attorneys were getting guilty clients off by tarnishing the victim's character (dressed a certain way, alone at the bar, etc.). Defense lawyers were exploiting outdated and oppressive cultural values, the burden of which fell heavily on women. These distorted the process of assessing whether the defendant did something wrong or not.