The people had begun to live according to the commandments. Although “they did not walk any more after the performances and ordinances of the law of Moses; but they did walk after the commandments which they had received from their Lord and their God,” they retained the promise of the land. The promise was that they would prosper if they lived according to the commandments. They were doing that. They prospered. Mormon “proves” it by declaring that “the people of Nephi did wax strong, and did multiply exceedingly fast, and became an exceedingly fair and delightsome people,” and “there was no contention among all the people, in the land.”
Verse 11 notes that they were married. That was certainly true, but it is such obvious information that Mormon certainly didn’t suggest that this was something new. What he is indicating is that life returned to normal, and to normal activities, such as marrying, and being given in marriage.
Verse 11 is functionally similar to Luke 17:27, which speaks of Noah: “They did eat, they drank, they married wives, they were given in marriage, until the day that Noah entered into the ark, and the flood came, and destroyed them all.” Marrying and being given in marriage simply refer to life continuing as normal.