... is inadmissable in court?
Let's put this in perspective. Say I hear from your friend that you stole a million bucks from a bank. I take this to the police, they corroborate, and arrest you. At court, you are contending that since my initial statement was second hand, all the corroboration would be thrown out too, and you wouldn't be prosecuted?
Also just as a reminder we have precedent here with the impeachment of Federal Judge Walter Nixon. The Nixon event went all the way to the Supreme Court and in Unanimous Decision (with 4 separate opinions.) The court hold it was the rules of the Senate and not the traditional rules of evidentiary procedure and the traditional rules of the court system that apply to the Trial in the Senate.
And the current Senate Rules allow hearsay and second hand information with impeachment.
https://en.wikipedia.org/wiki/Walter_Nixon
https://en.wikipedia.org/wiki/Nixon_v._United_States
https://twitter.com/Popehat/status/1177948602242519040
https://supreme.justia.com/cases/federal/us/506/224/