I believe the explanation on this issue can be found on Genesis 18. As you shall see, Arab muslims are descendants of Ishmael, Son of Abraham and his concubine Hagar. Both Hagar and her son believed in God. However, it was Isaac, and not Ishmael, to inherit the Covenant.
Hence, the misunderstanding that, even nowadays, that muslims and christians share the same God. And It`s well likely it might have been so at the beginning - as Ishmael himself and his Mother were Christians.
Even though, we share some of the Doctrines of the Old Testament, such as the 10 Commandments, which can be found on the Koran as well, the god that the muslims have come to worship is just a copy of the real God. Because anybody who fails to recognise -and to aknowledge - God and Jesus Christ as one, cannot claim to know -nor to worship- the one and only God.
Ive done some searching and haven
t found yet anything related to the Pope making such a statement. In any case, we share the same God with Protestantism and with any other Christian religions that regard Him and Jesus as one. Obviously this lad has no idea what he is talking about,so dont let him get to ya.