An impediment to articulation prevents or restricts the production of sounds and letters. An inward impediment would instead concern an absence of the capacity or reality of meaningful speech. Treating both as identical builds an invalid argument against inner speech.
Someone unable to speak because of illness may still communicate in writing; the failure of the vocal instrument does not remove intelligible content.
No impairment, inability, or bodily obstruction is attributed to God. The distinction is used only to analyze the theological terms carefully.