This is a nice exposition, but it would have been more clear if they laid out the difference between inertial and gravitational masses. So far, these two varieties of mass are equivalent in all our observations, but they need not be so. Negative inertial mass is pretty weird, as the examples illustrate; but negative gravitational mass (i.e., normal and negative mass repel according to inverse square law) would be something exciting to observe.

See, e.g., https://physics.stackexchange.com/a/8616