I mean why wouldn't he?

If he's telling the truth, then I would do the same.

If he's lying, then it might work, I would also probably do the same.

The interesting thing here is that two publications allowed the journalist to call Kushner "corrupt". This means that the lawyers would have seen compelling evidence that would mean one of two things:

1) Kushner wouldn't take them to court, because he would have to settle out of court in an expensive way

2) Kushner, if he were to take court action, would not win civil case.

This leads me to think that there is more than a shred of evidence, but an awful lot of evidence that he didn't declare what he was supposed to (at best) and at worse laundered foreign cash for his own purposes