What a terrible, misleading headline. As expected, the reference to the human brain is there entirely for virality reasons.

Whenever charge flows, there will be magnetic fields. It's common for this to occur, because all known life uses ion channels in some way, and multicellular organisms do use electrical potential for internal signaling (such as nervous systems). Thus, the study of magnetic fields can be used to gain information about the state of an organism and scientists are now applying this to plants as well.