The collection certainly contains the most essential teachings of Buddhist thought.
However, to say that the collection is the earliest section in SA/SN is not just based on the most essential teachings. It is mainly based on the three-anga structure (sutra, geya, vyakarana angas) of SA/SN, and the sutra-anga was the earliest of the three angas, according to Yin Shun.
This historical view on the formation of EBTs is completely different from the Pali tradition, which the nine-fold aṅga made up the entire Nikāyas of the Sutta-piṭaka was established at once in the first council.
Cf.: