Hi svahos,
The easiest way would be to animate as the tutorial suggest, then export as Alembic [abc] file. When all that is re-imported, the Alembic offers an offset parameter. Set the Offset to the values needed to reverse the animation. IF the file had 90 frames, then the Offset is at frame zero -90 and on end 90. See the example file [01] here.
https://www.amazon.com/clouddrive/share/66uCLLkRNrIaHYE4RgLbsW8w2BLgCXCPjawlzwYcxxT
The Freeze Layer has no shrink (in opposite to Grow) option. This needs an entirely different approach. Since the Grow effect starts from the Spline, it continues until all is 100%. This final position does not need a definition, it is reached automatically. If the reverse is needed, where should it start? This requires some kind of definition (Spline Mask, perhaps?)
Besides, at the moment, I’m not clear if you want the effect precisely in reverse, outside (most significant distance from the Spline) polygons fly in first, then close the area around the Spline, or grow from the Spline?
Cheers