The doubt is not silly: a true prediction, by itself, does not automatically prove every religious claim attached to it. But the "deceiving angel" argument only works if one first assumes the Christian gospel it is trying to defend.
The strongest form of the challenge
The argument says: Deuteronomy 13 warns that a prophet may show a real sign and still be false if he calls people to another god. Galatians 1:8 says even an angel from heaven should be rejected if he preaches a different gospel. So, even if Prophet Muhammad (PBUH) made accurate prophecies, that could only show that a powerful unseen being was involved. It would not prove that the being was from God.
That is the strongest version. It grants the prophetic evidence, then says: "Evidence of power is not evidence of truth."
Deuteronomy does not fit Islam
Deuteronomy's test is not simply, "Does this later prophet disagree with my community's interpretation?" Its stated issue is worship: if someone leads people after "other gods", reject him. Islam did the opposite. Prophet Muhammad (PBUH) called Arabs away from idols, angels, saints, tribal gods, and deified beings to the one God of Abraham, Moses, and Jesus.
Say, "We believe in Allah and in what was revealed to us and what was revealed to Abraham, Ishmael, Isaac, Jacob, and the Descendants, and in what was given to Moses and Jesus and the prophets from their Lord. We make no distinction between any of them, and to Him we submit."
Quran 3:84
So the Deuteronomy objection must prove that the God of Islam is "another god", not merely assert it. The Quran describes God as the Creator, the God of Abraham, the One without partner, neither born nor begetting. That is not a pagan rival to the God of the prophets. It is the strictest possible monotheism.
Galatians is not a neutral starting point
Galatians 1:8 has force for someone who already accepts Paul's gospel as the final standard. A Muslim does not. Nor is it a neutral rule agreed by all prophets that any later message differing from Paul's theology must be false. That would settle the debate by assumption.
Islam's claim is that God sent revelation through Gabriel, not a random spirit, and that the Quran came as a criterion over previous disputes.
And indeed, it is the revelation of the Lord of the worlds. The Trustworthy Spirit brought it down upon your heart, that you may be of the warners, in a clear Arabic tongue.
Quran 26:192-195
Muslims also do not accept that Islam contradicts Jesus. It contradicts later claims about Jesus: that he is divine, that his death is the saving center of faith, and that his resurrection theology is the measure of all revelation. The Quran honors Jesus as Messiah, born of Mary, a prophet, a sign, and raised by God - but not as God or the son of God.
What prophecy evidence does and does not prove
Accurate prophecy alone does not prove Islam in isolation. A person should not accept a prophet merely because of one prediction, dream, marvel, or impressive event. The classical approach is broader: the Prophet's (PBUH) truthfulness, the Quran's message, its preservation, its moral and legal wisdom, its uncompromising worship of God alone, its knowledge of earlier revelation, and the signs God placed in his life all support one another.
The "deceiving angel" theory remains logically possible in the thin sense that many skeptical theories are possible. But it is weak as an explanation. It asks us to believe that a devilish source produced a scripture that condemns Satan, destroys idolatry, commands prayer, charity, chastity, justice, mercy, care for parents and the poor, and devotion to the Creator alone.
O mankind, worship your Lord, who created you and those before you, that you may become righteous.
Quran 2:21
So, plainly: fulfilled predictions are not the whole proof. They do not force belief on someone who has already made Paul's gospel unfalsifiable. But they do count as real evidence when considered with the Prophet's (PBUH) life and the Quran's call to pure worship. The counterfeit theory explains less than it assumes.