. “The plan of salvation also includes those who acknowledge the Creator," --> God has a plan of salvation -- and that is Christ's sacrifice.
" in the first place amongst whom are the Muslims; " --> NOTE: this is AFTER Christians and Jews who are referred to in earlier chapters, all Christians of various stripes and Jews are much closer to each other than we are to hindus or muslims etc. Among those who are not in the Judeo-Christian tradition, you have Moslems, avowed monotheists. hence by the virtue of them acknowledging a one god, they are closer to Judeo-Christian tradition than polytheists or athiests
"these profess to hold the faith of Abraham, " --> they "profess", that doesn't say anything if they do or don't.
"and together with us they adore the one, merciful God, mankind’s judge on the last day.”" --> They adore the one, merciful God, but they are flawed in that their worship is directed to Satan, Allah. They may adore God, but they do not realize they are not worshipping Him.
It is noteworthy that Pope Gregory doesn’t say that the one God that he and King Anzir both worship is the same God. All he says is that both he and Anzir worship one God; in other words, they’re both monotheists. Beyond that, nope, not the same
The same may be said of Jews, of course: they, along with Muslims, reject the Trinity, the Incarnation, and the divinity of Christ, and yet clearly we Christians and Jews worship the same God.