shabbat songs - Why did the Vilna Gaon not approve of singing the zemiroh “Tzur Mishelo” on Shabbos?

I understand that the Vilna Gaon did not approve of singing the zemiroh “Tzur Mishelo” on Shabbos because it could be valid as Grace after Meals.

But on Shabbos, Grace after Meals is not valid without including “Retzai”; there is no mention of "Retzai" there, so why did he not approve?