Rashi considers the fact that neveila and tanur are mekabel tumah without touching, a kol dehu, regarding the issue of their yad being motzi tumah, as the Lev Aryeh explains in answering the Maharsha's question. But if so, what's pshat in the gemara just before this, that said ma l'zraim shekein tumasan meruba, and Rashi explains that they are mekabel tumah even from vladei tumah. According to the later Rashi, this would also seem to be a kol dehu, because we're discussing a yad being motzi tumah, so the fact that zeraim can accept tumah more easily is not a relevant chumra. But here it can't be a kol dehu, because the gemara said on 116a that by chada m'chada only kula vechumra parchinan, and not a kol dehu. And here is a case of chada m'chada, as the gemara goes on to say chada m'chada lo asya...
Yaron Barach, Brooklyn NY
Yaron, my apologies for the delay in answering your question.
1. Apparently, one must distinguish between the Halachah that the Tanur and Neveilah received Tum'ah without touching, and the Halachah that Zera'im received Tum'ah from a Vlad ha'Tum'ah. The difference is that in the case of receiving Tum'ah without touching, the fact that the Tum'ah went in so easily is no reason for it to leave easily (i.e., through the Yad). As the Lev Aryeh writes in his answer to the Maharsha's question on Rashi, the fact that the Tum'ah entered so easily shows that a mere Yad would not have the strength to take out the Tum'ah. Therefore, Rashi calls the Halachah of Kabalas Tum'ah without Negi'ah a "Pircha of Kol d'Hu," because there is no such logic that just because the Tum'ah entered easily, it should also leave easily.
2. In contrast, the logic of "Mah l'Zera'im she'Tum'aso Merubeh" is not considered a "Pircha Kol d'Hu." This is evident from the fact that he writes that specifically above, where it was "Chada mi'Chada," did we say "Mah l'Zera'im...." This means that since the Gemara (beginning of 116a) says that for "Chada mi'Chada" we do not ask a "Pircha Kol d'Hu," the Pircha of "Mah l'Zera'im" is not a "Pircha Kol d'Hu."
3. I suggest that the reason why "Mah l'Zera'im" is not a Pircha Kol d'Hu" is that the Tum'ah does not enter Zera'im in a particularly easy way. This is not similar to Tum'ah entering without touching. If Tum'ah enters without touching, this means that there is a qualitative difference in the way the Tum'ah entered; it entered with such ease that this may mean that it is especially easy for Tum'ah to enter this object (a Tanur). In contrast, when a Vlad Tum'ah enters Zera'im, this is a quantitive difference. There are more varieties of Tum'ah that can enter, but they do not enter by any different method. Therefore, the fact that more types of Tum'ah can enter Zera'im does not mean that there is less of a reason for why they should be able to go out via the Yad.
Kol Tuv,
Dovid Bloom