Either the person who tweeted was wrong, or you misunderstood them.

 
“One limitation to the president's power to grant pardons is ‘in cases of impeachment.’ This means that the president cannot use a pardon to stop an officeholder from being impeached, or to undo the effects of an impeachment and conviction.”

 
Which means an impeached person could be pardoned, but the pardon won’t undo the impeachment.