First let us quickly review their argument. The Gemara's question is that the Beraisa did not have to establish a second case of "Tehoros b'Tehoros" in order to cite a case where we are stringent, as we could have qualified the original case of "Nimucha" and stated that by "Lo Nimucha" we are stringent. According to the first explanation of Rashi, the Gemara's answer is that the Beraisa wanted to state a case of "Tehoros b'Tehoros." This is because normally people would think that Tehoros b'Tehoros is Batel, as is apparent from the Mishnah in Terumos (5:3). The Beraisa therefore cited a stringency regarding Tehoros b'Tehoros.

Tosfos' answer is that by stating a case of "Tehoros b'Tehoros," the Beraisa also implies that we are stringent by Lo Nimucha when the case is "Temei'ah b'Tehoros." Tosfos' reasoning is, if we are stringent regarding a mixture which is not even forbidden to everyone, we will certainly be stringent in a mixture that is forbidden to everyone.

It seems that Rashi did not think that the logic of Tosfos was correct. After all, Rashi (81b, DH "Divrei ha'Kol") employs the opposite logic by Bitul, stating that when the entire mixture is permitted to someone, there is in fact less reason to permit the mixture to everyone, as it can be eaten by someone without Bitul (even if it must be sold at a loss). Note that Tosfos (ibid.) does not explain the Beraisa in this fashion, indicating that he seemingly did not agree with Rashi that there should be less reason to permit this mixture (especially when it will cause a financial loss). This piece of logic seems to be the crux of their argument.

All the best,

Yaakov Montrose