Perhaps the explanation is - The Gemara explains its considerations as to why it might not be used for PPChamor - maybe it's deemed to be Basar BeDikula, on the other hand, maybe since it runs about it is a proper animal for PPChamor. Clearly the Gemara sees that PPChamor does not require more than that it is not Basar Bedikula. It does not require that it be an animal in every respect. Even according to R Gershom's approach, the sheep is certainly not Basar Bedikula, so obviously it may be used for PPChamor.

See below what we wrote in Insights to the Daf -

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INSIGHTS TO THE DAF- CHULIN 74

(b) RABEINU GERSHOM, however, suggests an entirely different approach. He explains that Rebbi Meir considers a fetus to be an independent animal only with regard to Shechitah. With regard to Chelev and Gid, or Revi'ah and Kil'ayim, he agrees that the fetus is not considered a separate entity. Rather, it is part of the mother, and thus these laws do not apply to a fetus.

How, though, does Rabeinu Gershom understand the Gemara later? The Gemara says that according to Rebbi Meir, since a fetus inside a slaughtered cow requires Shechitah, it obviously may be used for the Pidyon of a firstborn donkey, since it is a separate animal. According to Rabeinu Gershom, Rebbi Meir considers it an animal only with regard to Shechitah, but not with regard to any other law!

Perhaps Rabeinu Gershom means that only while the fetus is still inside of its mother is it not considered a separate entity with regard to the other laws. He agrees that once it exits the mother's womb, it is like any other animal. The Gemara that says that a fetus may be used for Pidyon is discussing a fetus that has left the mother's womb. (M. Kornfeld)