Reply buttons are only added for comments that have a signature with a date and time. If you replace {{Unsigned|janbery}} with {{Unsigned|janbery|22:08, 3 August 2020}} on that page, that comment will get a reply button (I added the "Example 2" section to demonstrate).

We can't add reply buttons to signatures (or "unsigned" templates) without the date and time, because there is no reliable way to distinguish them from user mentions.

My bad, template {{Unsigned}} on enwiki should be substituted. But template {{Nepodepsáno}} on Czech Wikipedia may be/is used without substitution on more than 3,7k talk pages.

Is this the same as T250516: Reply tool doesn't always work on signatures from the {{unsigned}} or {{user link}} templates or T252058: Support timestamped signatures generated by "unsigned" templates that haven't been substituted?