[erbium]

At 2:11 you can see it moving in a straight line.

[superkuh]

Not shown: all the momentum imparted by the launch itself.

[thinkcontext]

For the Nature paper they compared unpowered vs powered flight. Unpowered got about 10m, powered got to 60m, the length of their test facility.

[tzs]

In that clip it is not slowing down, showing that the ion drive is generating sufficient thrust to sustain flight. It's irrelevant that it used assistance to initially reach that speed.