Any of the justices can write concurring opinions; and I think the author was suggesting that there could be cases where an opinion authored by Justice Thomas garnered more support than the assigned or intended 'majority' opinion, controlled by Chief Justice Roberts. Anyway you look at it, the Chief Justice may be less likely to side with the liberals on the court, if the conservatives (led by Justice Thomas, who has seniority) can ignore him and write their own majority opinion...
The premise is actually spelled out in the article:
“But if the Chief Justice sides with the liberals, and winds up in the minority of a 5 to 4 case, the senior Justice in the majority would be Clarence Thomas . . . and Justice Thomas is by no means a shrinking violet.”
Thus, my comment, in order to avoid allowing Justice Thomas the right to assign the writing of the majority opinion, roberts would switch his vote to the majority, so that he would control the writing of the opinion, and thus, have it watered down.
There may be fewer 5 - 4 opinions as a result. But less bold, as well.