"extends" and "implements" by themselves are keywords, while "extends SuperClass" and "implements MyInterface" are examples of an extends clause and an implements clause, respectively.
From an English grammar standpoint, these aren't really clauses, as they don't have subjects. (Not included as part of the "clause" anyway.) But this isn't the first time existing terminology has been distorted in the making of a language specification.
I suppose we can think of "extends SuperClass" as shorthand for "...which extends SuperClass" - which
would be a clause, in English.