Shmuel only said explicitly מחזי כריבית by שבח, what about by פירות? This is aמח' ראשונים and affects how we understand the Gemara's question from the ברייתא of לאכילת פירות כיצד.
The רשב"א says that just like by שבח there is מחזי כריבית there is מחזי כריבית by פירות as well because he didn't actually buy anything. Therefore, the question from the ברייתא of לאכילת פירות כיצד is as was explained in the shiur.
However the תוס' הרא"ש says that by פירות there is no מחזי כריבית because by פירות the פירות are a tangible asset and it is clear that he is being paid for them. Only by שבח it is מחזי כריבית because the שבח is intangible and therefore looks like ריבית. Because of this (and other difficulties עיי"ש) the question from the ברייתא can't be a question on ר' נחמן as Shmuel would agree by פירות that the לוקח can collect the money because it is not מחזי כריבית. Therefore, he has a completely different pshat in the Gemara's question.
The Gemara is asking as follows. The Gemara thinks that the ברייתא is a proof to ר' נחמן, because the ברייתא has 2 distinct cases פירות and שבח. If a לוקח מגזלן got שבח then the ברייתא should have combined פירות and שבח into 1 case as they are the same דין. The fact that it split it into 2 cases is a proof that they are 2 distinct cases (בע"ח and גזלן). It turns out that the Gemara's question is actually on רבא, what is the chiddush of the 2 cases if they are both by גזלן, it should be the same din that he gets the money and the ברייתא should have combined them into 1 case. רבא answers with a different אוקימתא and ultimately, the chiddush is that even though there was a גזר דין on the קרן we don't say there was a קול on the פירות.