I would guess that the initial cooking did not kill all of the spores and the cooler temperature overnight allowed the organisms to grow. It is not the spores or the organisms themselves that cause illness. It is the by-products of metabolism of the c.botulinum that makes people sick.
I would never keep ANY food at room temperature for more than an hour but usually I would refrigerate it before that.
I think that this is probably a rare occurrence caused by a "perfect storm" of conditions and it does not mean that cooking potatoes in foil and then eating them immediately would be a problem.
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