Here goes again. I just wrote a lengthy explanation as to why you don't patch sidewall but when I hit submit, I got a DNS lookup error and all is lost.
In the days of bias ply tires, you could patch the sidewall and get away with it. That is because the strength in the sidewall was omnidirectional. In radial tires, most of the strength is unidirectional. A small break in the sidewall weakens the already weak longitudinal direction. Its like a wood board, it breaks easier with the grain as opposed to against it.
Since your thorn was close to the tread, the sidewall still benefited from the proximity of the belts. If the thorn had been further up the sidewall, the repair would not hold and the tire would fail. It would be dangerous to drive but I think you lucked out this time.