Following the sunnah is itself sunnah since the Prophet (s) himself followed the sunnah of past prophets in certain situations.
The Prophet (s) didn’t say things like, he was an Injeel-only or a Tawrat-only Muslim and on that basis disregard the normative example of past anbiya. And he certainly didn’t tell his ummah to be Quran-only Muslims.
Someone who has a Quran-only view of revelation might read the Quran and think, why is Muhammad (s) mentioned so infrequently? Why are other prophets and their stories so abundant in contrast? This ought to strike such people as a curiosity since Muhammad (s) was the final messenger- why should the final message have so little to do with the final Messenger?
Obviously these people don’t recognize asbab al-nuzul and all the Hadith literature which proves how tightly interwoven the verses and suras of the Quran are with the life and example of Rasulullah (s). By rejecting Hadith as fabrications, they are also, in effect, rejecting the Quran.