It states what happens for “less than 90 days”, and what happens for “more than 90 days”, but leaves what happens for “exactly 90 days” overstay as being unspecified.
If I understand your post correctly, then sadly for me, Phuket immigration didn’t follow that guideline. And in so doing, denied me an extra two months...
My extension of stay was granted on 28 August (after being under consideration from 29 July) and was stamped with an extension of stay only until 29 July 21.