Rashi explains the reason that cooking by exposure to the sun’s rays is permitted since it is not ‘derech bishul’ (ordinary method of cooking). The Gemara tells us that according to R’ Yosi cooking with Tiberian hot-springs is considered an issur min hatorah since they are heated by the fire of Gehinom. Why isn’t that too considered not ‘derech bishul’?
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Using the sun’s heat is not Derech Bishul, and using the heat of a fire is Derech Bishul, regardless of how one attained the heat of fire. The task is being done with all the right ingredients.
Just like we understand that wearing strange clothing doesn’t render any Melacha not Derech Melachto, here too, if the fire is in a strange place that doesn’t change the Melacha. The Melacha is as it should be.
We find this also later in the Masechta, that tossing a hard piece of earth at a tree to shed its fruit is not Derech Kotzer. However, using a knife that you would ordinarily not use is still considered Derech Kotzer.