Biden looked like an immature fool for his comment, so what does Trump do? Gets right down there in the mud with Biden, demonstrating he's equally immature. What an idiot.
And I suspect the Biden comment was actually Biden baiting him in the hopes he would reply, as he has, and thereby provide some cover if this Peter Schweizer thing gets any traction. Now if the DOJ goes after him, he can claim it's just Trump vindictiveness.