Everything about the commas is wrong. Everything. The sentence has two commas where they emphatically do not belong and one missing from where a comma is needed.
How to tell if you should use a comma between adjectives: Both dark and business are adjectives here. However, they’re not coordinate adjectives; you couldn’t reverse the order of the adjectives and have the sentence make sense. You can’t say They were dressed in business, dark suits. Nor could you join the adjectives with and (They were dressed in business and dark suits). The same applies to some and unknown; you couldn’t say the explanation was fictional to some and unknown degree or to unknown, some degree.
In addition, the sentence is compound (you could make it two sentences by removing the conjunction and), so it needs a comma after suits.
They were dressed in dark business suits, and the explanation they gave for the evacuation was clearly fictional to some unknown degree.
(This example didn’t come from a writing prompt on Pinterest, the way most do. Instead, it’s a close paraphrase of something read while doing research for a work of fiction. At this time, Mercenary Proofreader would like to suggest to conspiracy theorists — although their ideas are usually only hypotheses, at best, and sometimes just unsupported conjectures — that they’d have a slightly better chance of being taken seriously if they would learn how to write about their ideas in a grammatically correct manner.)