The Ran asks how we could permit a nadur item which is mixed into other items – after all, it’s a davar sheyesh lo matirin (through hataras nedarim), which is never batel!
The Ran resolves the problem by suggesting that a davar sheyesh lo matirin can be batel in a mixture of non-like (eino mino) items.
One could present a couple of other answers for the Ran’s question:
1. Ramban to Pesachim 30 (in Milchamos) says that the chachamim rule that davar sheyesh lo matirin is batel.
2. The Meiri to Chullin 97a says that an item is only a davar sheyesh lo matirin if the ultimate permitted status is a guaranteed result (he says this to explain that an egg from a safek tereifah isn’t a davar sheyesh lo matirin, even though the mother might prove not to be a tereifah).
Also worth noting: The Ran explains that we are machmir by davar sheyesh lo matirin in a mixture because the issur isn’t permanent, and so it is like min b’mino. Rashi in Beitzah 3b seems to say that the reason is more simple: Why eat it in a prohibited manner, if you could eat it in a permitted manner?
The Rosh and Ran disagree whether the challenge at the top of 52b is for Rabbi Yosi alone or also for the Rabanan. This appears to be tied to their debate about what Ashishot are – whether they are processed lentils (Ran) or the dross from lentils (Rosh on Mishnah 53b).