LSJ give this form as the perfect of ὁρμάω: pf. “ὥρμημαι” S.El.70, E. El.340, Th.6.33
, etc. ὥρμητο is the pluperfect, formed by adding augment (which doesn't change the initial long vowel, as Smyth indicates) and a secondary ending directly to the perfect middle stem ὥρμη-.
If ὥρμητο were imperfect, formed by an unusual contraction of ά + ετο > ητο, the eta would have a circumflex accent because the accent on the uncontracted form would fall on ά, following the normal rule that on finite verbal forms the "recessive" accent recedes as far back as possible--to the penult if the ultima is long and to the antepenult if the ultima is short (as it is here, since the final o micron is short), but no further back than the antepenult
Smyth 171-2 explains what happens to the accent in contracted forms:
171. Contraction.—If either of the syllables to be contracted had an accent, the contracted syllable has an accent. Thus:
a. A contracted antepenult has the acute: φιλεόμενος ῀ φιλούμενος.
b. A contracted penult has the circumflex when the ultima is short; the acute, when the ultima is long: φιλέουσι ῀ φιλοῦσι, φιλεόντων ῀ φιλούντων.
c. A contracted ultima has the acute when the uncontracted form was oxytone: ἑσταώς ῀ ἑστώς; otherwise, the circumflex: φιλέω ῀ φιλῶ.
N. 1.—A contracted syllable has the circumflex only when, in the uncontracted form, an acute was followed by the (unwritten) grave (155, 156). Thus, Περικλέὴς ῀ Περικλῆς, τι_μάὼ ῀ τι_μῶ. In all other cases we have the acute: φιλὲόντων ῀ φιλούντων, βεβὰώς ῀ βεβώς.
N. 2.—Exceptions to 171 are often due to the analogy of other forms (236 a, 264 e, 279 a, 290 c, 309 a).
172. If neither of the syllables to be contracted had an accent, the contracted syllable has no accent: φίλεε ῀ φίλει, γένεϊ ῀ γένει, περίπλοος ῀ περίπλους. For exceptions, see 236 b.
Since ὥρμητο is accented on the first syllable, the η cannot be a contraction of α + ε. That would imply an uncontracted form in which the accent had receded to the ante-antepenult (pro-proparoxytone), an impossibility in Greek: *ὥρμαετο
Maybe there is another possibility: that for hundreds of years editors of the text have been accenting ὡρμῆτο incorrectly, failing to recognize the irregularly contracted imperfect.
But I'd sooner accept ὥρμητο as "vivid" use of the pluperfect: "he had set himself in motion" = "he was on his way to help the Chians" or "he was off to help the Chians." The imperfect seems less appropriate here: "He was setting out . . . " If the emphasis were on "setting out", rather than "being on his way", the aorist would seem more appropriate, but ὥρμητο is clearly not aorist.