The claim: "Acts like the prayer were carried to later generations by unbroken practice (tawātur). That is a reliable source outside the Qur'an; so the religion does not run on the Qur'an alone."
The guarantee of transmission is tied to the text
- "It is We who sent down the Reminder, and We will surely preserve it." (15:9)
- "Its collection and recitation are upon Us … then its clarification is upon Us." (75:17-19)
The preserving and clarifying of the text belong to God; the guarantee of transmission is tied to the text, not to a source outside it.
Practice = the living-out of the text
The prayer continuing across generations is the enactment of the worship the Qur'an commands. It need not be a source of rulings added to or against the Qur'an; it is its application.
An honest limit
Fact: the practice (e.g. prayer) was transmitted across generations. Inference: "this is a binding source of legislation outside the Qur'an." This second part is an interpretation — treating practice as the application of the text is also a valid reading. Where the binding force of practice-details (e.g. rakʿah counts) rests is a matter of interpretive dispute; we leave both readings as they are.
Source: Qur'anic verses (M. Okuyan meal). Presented with a text/interpretation distinction.