I have never said “the SN/SA is the only true teachings of the Buddha”.
But I certainly consider Buddha Dhamma is essentially found in the major collection of the SN/SA suttas.
That is, the major collection of the Pali SN and its Chinese counterpart SA is mainly about knowing (jānāti) and seeing (passati) the four noble truths, the notion of anicca, dukkha, anatta (or anicca, dukkha, suñña, anatta), and the middle way, which all are the core teachings of Early Buddhism. E.g. see the following book by Choong Mun-keat:
The Fundamental Teachings of Early Buddhism: A Comparative Study Based on the Sūtrāṅga portion of the Pāli Saṃyutta-Nikāya and the Chinese Saṃyuktāgama (Series: Beitrage zur Indologie Band 32; Harrassowitz Verlag, Wiesbaden, 2000).
I also consider, according to Ven. YinShun, SN/SA is an early compilation, whose structure had its its origin in the first Buddhist council. SN/SA was the foundation of the four principal Nikayas/Agamas in the formation of EBTs, although the extant four Nikayas/Agamas are sectarian texts.