Here's why:
* Noun dependent clauses function as nouns within a sentence. They typically begin with words like "who," "which," "that," or "whose" and answer questions like "who?" "what?" or "whom?"
* This sentence has a subject ("Lord Albemarle") and a predicate ("was now the only advocate of Phileas Fogg"). The descriptive phrase "an elderly paralytic gentlemen" is a appositive – a word or phrase that renames or identifies the noun it follows.
Therefore, the sentence is a simple sentence with a compound subject.