The answer is correct, but is (perhaps deliberately) incomplete. Replying to recent comments is not disabled, there is an exponentially increasing timeout. Yes, you can always click on the "link" link and then hit "reply", but in-thread, the "reply" link only appears after a timeout that gets longer as the comments get deeper.

In summary:

+ "reply" on comments in old threads are disabled.

+ "reply" on comments in new threads have a timeout

+ the timeout is very short if the comment is shallow

+ the timeout gets longer as the thread gets deeper

+ the timeout can be circumvented by clicking "link" - then you are offered a "reply" link immediately.

+ PG might put the timeout out the shortcut method, but hasn't yet done so.