What you don't know is that this NPR article that uncritically repeats Muslim sectarian origin myths as fact.
So when you make arguments based on the idea that SunniMuslims existed in the 7th C, you are "accepting the beliefs of Muslims as evidence". That you are oblivious to what you are doing doesn't change the facts.
If you are interested in why the NPR article you approvingly quote is not actual history:
In recent years, scholars have increasingly used the label “proto-Sunni” to denote a
late Umayyad — early Abbasid period movement which played a pivotal role in the
formation of Sunni Islam... the category proto-Sunni usefully underlines the fact that, like other
religious traditions,5 Sunni Islam did not appear in history fully formed; but that it
emerged through a complex historical process, a process which yielded widespread
Sunni self-awareness no earlier than the late 9th century.6 As such, the designation
proto-Sunni underscores the provisional nature of the many competing versions of Islam
in this period, and against that background, the crucial role of those who prepared the
way for an eventual Sunni consensus... While much of the credit for the
“Sunni synthesis” must go to jurists and theologians of the 10th through the 13th centuries,
such as al-Ash‘arı¯ (d. 936) and al-Ghazza¯lı¯ (d. 1111), who absorbed many of the
cross-currents of early Islam into grand legal and theological schemes, it is important to
remember that those very schemes would have proven impossible without the
foundations laid by the early proto-Sunnis.83 Laying these foundations or plausibility
structures is their greatest achievement; an achievement which was the result of the
proto-Sunnis’ uniquely satisfying solutions to the historical pressures they faced on one
hand, and the increasing prestige they enjoyed on the other.
The Roots and Achievements of the Early Proto-Sunni Movement: A Profile and Interpretation - Matthew J. Kuiper