The title is incorrect on two accounts. 1) We've witnessed geomagnetic storming with many other probes, but never had multiple spacecraft separated on the spatial scale of the electron diffusion region (several to 10's of km's). Since this region where magnetic reconnection occurs is so small and spacecraft travel fast, you also need extremely fast temporal resolution to make worthwhile observations. 2) (this one is much more minor) Ever since MMS was launched, the sun has been relatively quiet. Solar Cycle 24 has been a total let down; the peak of activity was almost a factor of two lower than what model-based predictions were saying several years ago. Reconnection is a commonly occurring process, it only requires that the Interplanetary Magnetic Field (IMF) have a southward component.
Anyone with journal access can check out the Science write-up.
OK, after reading it twice more I have still a lot of details to research, but what intrigues me most is the configuration of probes. If distance is crucial and having at least four (I'm assuming that it's for similar relativistic reason why GPS works only if you have 4 satellites), why are they not arranging fifth one in the centre of mass of that tetrahedron? I don't need to study crystallography to know that it minimizes distance and massively improves probability of having at least four in the reconnection zone. Is there any reason other than sheer price of these satellites that would prohibit it?