I think a lot of what is being said here is speculation; conditions may have been VFR (even if marginal) when this pilot took off. Unless this gets further investigated and posted to the NTSB site, we may never know.

"Special VFR" (not a "special IFR ticket") is a clearance that a pilot with a current instrument rating may request when visibility is low (but still at least 1 mile) and some other conditions are met, including being in contact with ATC. It's often used to get out of an airport with low visibility (e.g. ground fog) but clear skies above and on the rest of the route.

As a private pilot under Part 91 regulations you can take off on an IFR clearance with visibility as low as essentially zero; it's your judgment call as a pilot.

In this case it sounds like neither of the above would have applied, since he wasn't in controlled airspace and likely not on an IFR clearance (though that's not even clear).

Anyway, sorry to hear about the accident, and glad folks like Martin are out there.