The other theories of formation (volcanism, subsidence, etc.) had been discredited by the late 1920s, leaving the meteor impact theory the only one left standing. And there was a lot of good evidence supporting it (finely pulverized rock under the crater floor, the meteoric iron under the crater found by drilling, etc.). There was no controversy about the crater's origin, had been none for decades, when Shoemaker found the polymorphs. I know, because I read the original published literature - from the teens, twenties and thirties, Shoemaker's paper on Meteor Crater, AZ, his subsequent paper where he showed that the Ries Basin was an impact feature due to the polymorph presence (this was the first real case where it was crucial in making the determination), and also Shoemaker's obit in Science, which does not assert that he proved the nature of Meteor Crater.
Your geology department field trip may have been passing along a "good story" rather than a critical examination of the literature.