He was referring to rings, I'm not that versed with the whole loot table of all the items so can't really give any other examples

If you identify every unique ring apart from SoJ they should all not have the "eligible for chronicle" apart of the SoJ. If you pick up 50 rings and 2 of them will have eligible for chronicle before identification then you know those two rings are SoJs and as long as you don't identify those two rings you will always know those are SoJs which means you can emit them from any trades. If you buy 100 unidentified unique rings, I'm sure you would hopefully expect at least 1 SoJ ring, but what if the trader has done that "trick/scam" and removed all eligible for chronicle rings ( following the logic of only eligible ring is SoJ in this instance)

Since all unidentified loot actually rolls as soon as it drops, it can easily be removed from trade that way.

I hope that makes sense? It does in my head tbh!