The url works for me, and points to my comment explaining how to show the bottom bar. Pasting the relevant text here:
Find the username of the commenter in the upper left. This defines the height of the top-bar.
Long-press the whitespace in the top-bar to the right of the username. Make sure to hit white space, if you tap the username or one of the other linkable elements of the top-bar something else happens.
If you have tactile feedback enabled on your phone, you’ll feel a vibration and/or hear a click as the bottom-bar appears. If the comment is lengthy, the bottom bar may have appeared off screen… and if you have tactile feedback completely disabled you may get no indication this has happened. Now that the bottom-bar is visible you can click the reply icon.
The url works for me, and points to my comment explaining how to show the bottom bar. Pasting the relevant text here: