Church weddings were the exception rather than the rule until about the turn of the 19th/20th centuries. It was customary for a wedding ceremony to be held in the bride's parents' home. If, however, she had no parents or she was getting married a long way from their home, she might get married in her employer's house, or in the manse, which is the minister's house.
As there is evidence to confirm that Rev Robertson lived at this address, yes, your relative got married in the manse.