Impeachment is not a criminal process. Its sanctions are limited to removal from office (thanks to the memory of Charlie I, no doubt!).
The power of pardon is over the federal criminal process.
Still unresolved is, can a president pardon himself (but that would be wrong, LOL) for criminal offenses committed while in office?
In case of impeachment, they can kiss their pardons goodbye.
Just little crack in the dam. Nothing to worry about.