Sedley says flatly that "Katastematic pleasure is the absence of pain."
Both are correct, and it may be more helpful to highlight the similarities in the interpretations.
The "ste" in Katastematic is the same root that gives us "static" in the sense of "standing" or "still" -- and the "kata" is an intensifier! Etymologically the whole word means "characteristic of thoroughly standing"
As we have seen, Epicurus does literally say that "Katastematic pleasure is the absence of pain" at DL 10.136 where he says "Undisturbedness (ataraxía) and non-suffering (aponía) exist as established (katastēmatikái) pleasures, but joy and merriment are seen from movement through activity."
[Mental] painlessness/non-suffering (ἡ ἀπονία) is equated here with a katastematic (established) pleasure.
But this leaves plenty of room to also agree with Austin:
katastematic pleasures are sensory pleasures that issue from confidence in one’s ability to satisfy one’s necessary desires and an awareness of one’s healthy psychological functioning
It is helpful to see painlessness/non-suffering as referring more to the mind/spirit and less so the body... but, of course, the close connection between the mind and body was never denied by our school in any way.
Consider Plutarch [Non Posse, 1089D]:
'See then, first of all, what they are doing: transferring either this "non-suffering" or "painlessness" or "stability" back and forth from the body to the soul – then back again from the [soul] into the [body]!'