roberts would just change sides to control the opinion to have it watered down.
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...