all of his [Mitt's] ancestors? never served in the military in any capacity?
If I recall correctly, Mitt's great-great-grandaddy, Parley Pratt, fought (sort of) in the failed Zion's Camp expedition organized for the purpose of punishing the citizens of Independence, Missouri for their destruction of a Mormon printing press.
Parley also served with the Danites (a.k.a the Army of Israel, the United Brothers of Gideon, the Nauvoo Legion and the Mormon Militia) in the Missouri Mormon War of 1837, and in the Illinois Mormon Wars of 1844 and 1847. In that capacity, he was active in the killing of Gentiles and the using up of his fellow Mormons to the point where he was captured by the Missouri National Guard and charged with treason, murder, arson, burglary, robbery, larceny, and perjury. Most reputable historians believe he would have been hanged had he not managed to escape from custody, along with Joseph Smith, by bribing the sheriff taking him to trial with a substantial amount of cash and whiskey.
Other of Mitt's ancestors slaughtered Paiute Indians in the Walker War (1853-54), Ute Indians in the Black Hawk War (1865-72), Gentiles in the Mountain Meadows Massacre of 1857, and the United States Calvary in the Utah Mormon War of 1857. They also participated dozens of other Indian conflicts.
While I think it is correct to say that Mitt's ancestors never served for the United States military, or for any state militia, they undeniably fought many times with distinction against these institutions. Therefore, the statement that Mitt's ancestors never served in the military in any capacity is overly broad unless given the above qualification.