Looking at the etymology, impeach is from old-french “empecher”, and “em” was an alternative form of the old-french prefix “en”.
And that “empecher” is itself is derived from the late latin “impedicare”, which uses the Latim prefix “im” from which the French prefix comes. And is prefixed to “predica”.
So it it is a prefix.
Of course, the latin (and French) prefixes aren’t used to indicate opposite meaning, like “in” often is. But that’s just yet more bullshit.