I think the Medium design team just want to create a reaction for readers to take when viewing an article. They set the clap feature as well as we clap in the real word.
They focus on the feeling of reader and what they are going to do when they are reading a great article. Reader probably clap or thumb the desk to show their excitement. It does not matter how many seconds the reaction will take and how many times they want to react. Just for a good experience for doing the clap reaction.
I think that is the reason the difference between web and mobile side. And the undiscoverable undo action.