That applies to both sides. We couldn't have impeached Obama for his pro-terrorist actions before the election. They cannot impeach President Trump (or Justice Kavanaugh) for any indiscretions from prior to the election/confirmation, even if one of the thousands of completely arbitrary allegations happens to be true.
1) Because it never happened.
2) Because there's no chance it would ever make it to court.
3) Because if it did, there's no way the state could ever get a conviction.
But in court of public opinion, with the support of Hollywood airheads, press whores, man-hating government witches, and complicit Republican lackeys, no evidence is necessary, and a fair judgment is a luxury.