Trade secrets are intellectual property that is not generally known to the public, confers economic benefit on its holder because the information is not publicly known, and where the holder makes reasonable efforts to maintain its secrecy.
The pages of "internal research" this person copied were Facebook's intellectual property, not simply "choices that they made". Judging from the complaint, it seems to have conferred an economic benefit. And Facebook has attempted to maintain its secrecy. This would appear to be a trade secret. To say otherwise would be a bit like saying I didn't steal a trade secret by stealing the KFC original recipe, because that's just some choices a Kentucky Colonel made...
Further, by the whistleblower's own admission, much of what Facebook was doing wasn't illegal. Certainly not the internal research that was copied. The only thing anyone is suggesting might be illegal is lying to investors, and the only evidence of that is "they lied to the public, so they must have.... right?"