I do not know about the fuel referenced above, which may be misnamed but chemically, isobutane is NOT a mixture or combination of propane and butane. There are two forms of the four carbon butane molecule possible, either a straight chain which is called n-butane and a central carbon with three pendant carbons, called isobutane. Both are butane, same number of carbon and hydrogen atoms, same molecular weight, but slightly different physical and chemical properties.
And no, butane does not have a lower boiling point (at 0 to 1 degree C) than propane (-42 degrees C).
Actually, three products are sold, propane, butane (actually a usually a mix of both isomers) and LP gas, a mixture of propane and butane.