You are correct, local_cdn_url
replaces a local URL with a CDN URL. And this is not even a local URL but a relative one.
So I think these would suffice instead of those UrlHelper
calls?
importScripts("<%= (Discourse.asset_host || '') + "/javascripts/workbox/workbox-sw.js" %>");
and
modulePathPrefix: (Discourse.asset_host || '') + "/javascripts/workbox",