I don't know the context surrounding the sentence so I can't give you an exact answer but I can think of 1 of 2 possibilities. The person could be saying the source of inspiration IS endless (like you said). Or they could be saying it's not endless and is just asking rhetorical question. For example, someone says "Nothing inspires me anymore, I will never have that spark again" and another person replies "How can you say that? How can the spring of inspiration be endless?"