Amphibian and semi-aquatic aren’t the same thing. Frogs are amphibians because they start as gilled tadpoles and turn into hoppy lungy frogs. Turtles are not amphibians because they hatch as a turtle, live as a turtle, and die as a turtle. It’s turtles all the way down.

What about the platypus?

Platypus babies only drink Bundaberg Ginger Beer. Also, they’re not amphibians.

Platypi are mammals

Platipy, like echidnas, are monotremes; “primitive”, egg-laying mammals.

Caecilians are amphibians, like frogs and salamanders. Whole different clade.