Well in order to make it work please "enable cache first strategy" from wp-amdin - vibebp-> settings -> service worker and change the service worker version, save the settings , then regenerate the service worker .
Then hit the download icon there and check if it works offline. .
Also note that the youtube videos will not be downloaded due to restrictions from youtube while the locally uploaded video in media library will be downloaded /
Ok I changed the version of service worker and then regenerate the service worker and caching started working : https://gyazo.com/f23752e7cd5f04a219ba0e233fe2284b
IT takes second page reload for the service worker to start cache things