You may fly a Lake Amphibian from a land airport without holding a seaplane rating. You may take a practical test for a certificate or rating in a lake amphibian without holding or testing for a seaplane rating, so long as you do not attempt to land on water.
The Lake Amphib (Buccaneer or Renegade) is an amphibian, not a seaplane, or a landplane. If flown from the land, a seaplane rating is not required. If flown from the water only, a land rating is not required.
If one desires to utilize the Lake for a practical test for the commercial certificate, the practical tests for both single engine land, and for single engine sea may be accomplished in the airplane, although technically as two separate checkrides.