The earliest non-Muslim reports do show the Arab conquests as a religiously charged political and military movement. They do not, however, show that Islam or the Quran was invented later; they mainly reveal what outside observers noticed, understood, and chose to record.
What the early witnesses actually say
The argument has real force if stated carefully. The Doctrina Jacobi, usually dated around 634 CE, reports that a prophet had appeared among the Saracens and connects his followers with warfare and hopes surrounding Palestine. It does not name Islam or describe the Quran in detail. The Armenian history attributed to Sebeos, written in the 660s, describes Muhammad (PBUH) as an Arab merchant and preacher who taught the descendants of Ishmael to recognize the God of Abraham, then united them in a movement that advanced toward the Roman lands.
Thus, these witnesses present an emerging Arab religious-political coalition, not a modern-style textbook account of a completed faith. Their silence about the word "Islam" and the Quran is a legitimate historical question.
Why this does not contradict the Muslim account
First, Islam did not claim to appear as a fully developed state from its first day. The Prophet Muhammad (PBUH) preached in Makkah for years before the migration, and the Quran itself describes revelation as gradual:
"Those who disbelieve say, 'Why was the Quran not sent down to him all at once?' Thus it is, so We may strengthen your heart with it, and We have revealed it gradually, in stages."
Quran 25:32
Second, a religious movement becoming a political and military community is not evidence that it was merely political. The Prophet (PBUH) was a preacher, judge, statesman, and military leader. The Quran itself addresses worship, law, war, treaties, charity, and communal government. A seventh-century observer focusing on armies would naturally describe the public and strategic dimension.
Third, the reports are external and limited. The Doctrina Jacobi is a polemical Christian work, and its information appears to be gathered amid confused reports during a crisis. Sebeos wrote decades after the events and summarized what he understood from the region. Neither author set out to catalogue Muslim scripture, liturgy, or internal documents. Absence from such texts is therefore not evidence of absence.
What the evidence does and does not establish
These sources do establish that, very early, outsiders knew of an Arab leader associated with monotheistic preaching, Abrahamic claims, and military expansion. That is significant independent evidence for the movement's early religious-political character.
They do not establish that the Quran was absent, that the movement lacked a defined message, or that Islam crystallized only under later rulers. For that conclusion, one would need positive evidence of later invention, not merely silence in brief and imperfect outside accounts. Early Quranic manuscripts, inscriptions, and the coherent internal text of the Quran fit a movement whose revelation and community formation unfolded together.
The fairest conclusion is that these witnesses illuminate early Islam from the outside; they do not overturn the central Muslim account.