I think the thief was accepted by the ewe because she had not bonded with her lambs. After bonding, she will not allow other lambs to nurse. The thief may continue to steal whenever and from any ewe it can; while the ewes are lined up eating grain from the troughs the thief will run down the line stealing where it can.
This year we’ve seen several ewes (close to delivery) trying to steal new born lambs of others ewes.
We take advantage of this strong mothering instinct. If we have a lamb get rejected by its mother, a weak lamb being out competed by siblings, or mother can’t produce enough milk for her lambs we will see if a new mother will accept another lamb. Some ewes are more accepting than others; we find it useful to have a few that are accepting.