Impeachment isn’t a criminal proceeding.
No, it isn't, but like an indictment it opens the door (yes I know an indictment is) to trial.
The articles of impeachment serve the same purpose.
Nor is the 'trial' held for those articles a criminal proceeding, but a determination of whether that person is fit to hold office, and upon conviction, they may be removed, with a prohibition on ever holding office again. That is not a criminal trial, which may stem from charges brought during impeachment proceedings, and is a separate issue.
Impeachment is seen as remedial, not necessarily punitive (like criminal proceedings would be, if found guilty).