It's important to compare the content in the final paragraph to the rest of the passage. It introduces new considerations (volcanic ash, variations in sunlight) that might also explain ice ages. True, it reiterates that Milankovitch's theory is testable, but in no way does it summarize the passage. The paragraph is saying "Here are some other possible explanations. Milankovitch's explanation (orbital variations) is being tested, but that doesn't mean these other factors may not turn out to be important." Therefore, the author is sounding a note of caution. Despite the promising results so far, we can't be sure that this theory is the answer.
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