You could say the same for Christianity, really - In the early Christian period it wasn't clear if Christianity was a new religion or a sect of Judaism. Many early Christians would have considered themselves Jews.
There was little clear, standardised definitions of simple things like whether Jesus was a man or a god, and to what extent he was divine, whether Jesus even existed or was a supernatural being are not cleared up for decades after Jesus supposedly died.
Then you look at the huge changes in how Christians view the scriptures throughout time - the Catholic interpretation of the gospels is radically different in say, the Crusader era than it was post-reformation. Much of modern Christianity's views on the bible are influenced by people like (Thomas Aquinas)[https://en.wikipedia.org/wiki/Thomas_Aquinas) who lived twelve centuries after Christ.
In fact, this is one of the main issues with Islam, because there is not this tradition of Scholasticism in Islam. Interpretations of the literal text of the Qur'an are almost verboten - There is almost no modern study of the Qur'an in this style and the majority of Muslims today will, for example, continue to insist that the Qur'an was written by Mohammed and is the direct word of Allah, something which is unlikely, whilst a Christian educated in theology will more than likely admit that the Gospels are written by a variety of anonymous human authors.
Judaism is similar - many texts now central to Judaism date from the beginning of the Christian period such as the Mishnah and Talmud.
So yes, whist Islam is a 'newer' religion in the literal sense where the texts are newer and the events they talk about are more recent, philosophically I think Christianity and Judaism are philosophically more 'modern'. Islam in practice retains this 'medieval' aspect while the other Abrahamic faiths have bent to allow for modernism.