The answer to that is from two perspectives:

First: the principle is that whatever is established in voluntary prayer is likewise established in obligatory prayer, unless there is evidence to the contrary. The conduct of the Companions, may Allah be pleased with them, supports this principle: when they mentioned that the Prophet ﷺ would pray on his mount while travelling, they said, “Except that he would not perform the prescribed prayer upon it.” (1) This shows that the default is for whatever is established in voluntary prayer to apply equally to obligatory prayer; hence, they needed to make an exception for the obligatory prayer. This hadith also indicates that, when mentioned without qualification, prayer encompasses both obligatory and voluntary prayer.

Second: greater latitude is afforded in voluntary prayer than in obligatory prayer. Thus, if no latitude is afforded for standing to the imam’s left in a voluntary prayer, then it is even more so not afforded in an obligatory prayer. This is the reasoning underlying the author’s statement.

Most scholars hold that the prayer is valid when one stands to the imam’s left while his right side is unoccupied, and that a single follower standing to the imam’s right is merely preferable, not obligatory. Our shaykh, Abd al-Rahman ibn Sa‘di, may Allah have mercy on him, chose this view.

They responded to the argument based on the hadith of Ibn Abbas by saying that this was a mere action, and a mere action does not indicate obligation. This is a principle of legal theory: the Prophet’s ﷺ mere action does not indicate obligation, for had it been obligatory, the Prophet ﷺ would have said to Abdullah ibn Abbas, “Do not do this again.” Just as he said this to Abu Bakrah when he bowed before joining the row. (2)