Is it the pope's private opinion that Muslims worship the same God?
The short answer
No. The Second Vatican Council taught that Muslims adore the one God — living, merciful, all-powerful, Creator of heaven and earth, who has spoken to humanity — and strive to submit to his decrees as Abraham did.
Not a private opinion
This is the teaching of the Second Vatican Council.
Nostra Aetate
The Church regards Muslims with esteem. They adore the one God, living and subsistent, merciful and almighty, Creator of heaven and earth, who has spoken to men, and they strive to submit wholeheartedly to his decrees, as Abraham — with whom Islam links its faith — submitted to God (Nostra Aetate 3).