The workaround in the article was:

    var thingsGroupedById = _.groupBy(things, function(t){
      return 'seller' + t.sellerId
    });
    // { seller1: [...], seller2: [...] }

But I actually would have failed this in code review:

    _.map(thingsGroupedById, function (things) {
      // ...
    });

and replaced it with:

    _.map(_.keys(thingsGroupedById), function (thingKey) {
      var thing = thingsGroupedById[thingKey];
      // ...
    });

because although mapping over objects is "normal", it has never been 100%.