I think it should be very clear that there were teachings not found in the suttas. The teachings are basic outlines, yet the teachings were very clear at the time just before the Buddha’s Parinibbāna. After all he asked just before that final attainment.
As pointed out by exven Aggadhammagavesaka in his video proving the commentaries were existent during the time of the Buddha (his very teachings) He mentions that the monks took turns going for alms while the others would stay behind with the Buddha. This was just after DhammaCakkaPavattana sutta so the others could attain.
The best a suttanta person can do is scrape the suttas for further definitions and then act all smug and say they “discovered” the definition based only on the Buddha’s words. . This is actually what the abhidhamma and commentaries do. The commentaries often quote suttas, but it seems that nobody reads these.
The teachings are all there and written down. Unfortunately, the commentaries were re-translated because they were lost. (I bet eaten by termites when left in storage or if orally, just lost by translation).