Very good question. Mohammad believed that he was a prophet in line with the other prophets - Abraham, Moses, Elijah, etc - and in the Koran one finds claims to this, as well as requests for the Jews and Christians to look to their books for evidence.
The Messenger believeth in what hath been revealed to him from his Lord, as do the men of faith. Each one (of them) believeth in Allah, His angels, His books, and His messengers. “We make no distinction (they say) between one and another of His messengers.” And they say: “We hear, and we obey: (We seek) Thy forgiveness, our Lord, and to Thee is the end of all journeys.” [2:285]
Then if they reject thee, so were rejected messengers before thee, who came with Clear Signs, Books of dark prophecies, and the Book of Enlightenment. [3:184]
They say: “Why does he not bring us a sign from his Lord?” Has not a Clear Sign come to them of all that was in the former Books of revelation? [20:133]
The original Muslim understanding was that the former scripture confirmed the Koran.
This is the Book; in it is guidance sure, without doubt, to those who fear Allah; who believe in the Unseen, are steadfast in prayer, and spend out of what We have provided for them; and who believe in the Revelation sent to thee,
and sent before thy time, and (in their hearts) have the assurance of the Hereafter. [2:2-4; emphasis mine]
This, however, changed later, obviously when it became clear the Koran did not compliment the Old and New Testament, and the claim that the bible is untrustworthy was brought forth. Thus, Muslim apologetics today is left with a circular argument where everything that compliments the Koran is true, and everything that doesn’t is thrown out. In essence, you bring forth scripture as your evidence, then admit the evidence used for your case is faulty - most lawyers would consider that suicide.
As I said, the consistency of revelation is a confirmation of what came before - the Koran is a confirmation of what came after. Why did God change His plans?
Have you ever by chance studied the belief of the early Church Fathers? If you read their earliest writings, you’ll find an unbroken chain of Trinitarian and Divine Christ belief from the earliest days of Christianity up to the 8th century. What men like Clement I or Saint Ignatius wrote at the turn of the second century is very similar to what Saint John Damascene writes in the 700’s. No where is it argued to any large degree until the coming of Mohammad.
Keep in mind that Mohammad said that the first few generations of Muslims would be reliable.
Narrated Abdullah:
The Prophet said, “The people of my generation are the best, then those who follow them, and then whose who follow the latter. After that there will come some people whose witness will go ahead of their oaths, and their oaths will go ahead of their witness.” [Sahih Bukhari, 3:48:820]
Imagine if I told you a companion of Mohammad was not reliable, despite this statement by Mohammad - now try to imagine how a Christian feels when a Muslims say that Clement I and Ignatius, who personally knew the apostle John who was the “beloved disciple” of Christ, are not reliable. It would not be consistent, and to Christians it’s displaying double standards.
She’s referring to the gradual canonization of what you know today as the Koran, which you found part of the story of on Wikipedia. In fact, Ibn Mas’ud, one of the most trusted companions of the prophet, claimed that the Koran held by Uthman was a corrupt version with surat that didn’t even belong there.

Even today the Shi’a Muslims claim there are certain passages of the Koran that Sunni changed.