He had developed the system closest to Bitcoin, he was actively seeking collaborators to turn his system into a practical offering briefly before Bitcoin was released, and he was the only cipherpunk who conspicuously said very little when the system he'd been trying to realize for a decade suddenly appeared. Satoshi credited all his inspirations except for the most obvious one, Szabo's. No one in the cipherpunks mailing list thought any of this was odd, probably because it was obvious to them who Satoshi was.
In contrast to a certain convicted Australian fraudster who got caught trying to backdate his statements, Szabo got caught trying to front-date them. His politics are a match to Satoshi (tbf. true of all the cipherpunks), his coding style matches Satoshi, his writing style matches Satoshi if you disable the British English spellchecker. For good measure his initials match Satoshi.
I view articles like these as a good test of which investigative journalists are hacks indifferent to the truth - except for that Wired guy, who I think knows better but thinks it's righteous to lie a little to protect Satoshi's anonymity.