The claim here was that "magnetic force decayed by 1/r³" which is the result of seeing a dipole ("two sources") from far away. Each pole decays by 1/r² and the net result (the combined effect of both poles on a test particle) is proportional to 1/r³

I would look at these:

https://physics.stackexchange.com/questions/184211/why-does-...

https://physics.stackexchange.com/questions/293310/why-does-...

And understand both electrostatic and magnetic dipoles in analogous terms.