Per the book that tells Feds how to give checkrides, he's wrong. The FAA has four class ratings for airplanes, listed above. You take a checkride in one of the four. Per the last part of the FSIMS above, the class ratings for aircraft not used in the checkride remain the same as they were on the old certificate.
The ONLY way I see to cover two classes would be to do the ride in an amphib and do both land and water operations. If you do a PBY-5 or G-111 type and don't do both, you are limited to what you've demonstrated. That is explained in AC 61-89.