My mental model is that when the tutorial was written there was one file, now TG4 have gotten more cautious and split it up into tiny one-second segments. Is this hypothesis right?
The Media Presentation Description (MPD) is a document that contains metadata required by a DASH Client to construct appropriate HTTP-URLs to access Segments and to provide the streaming service to the user.
Not quite. What you see is normal. Browsers look at MPD documents to know where to download video segments. They then play these video segments. You probably saw a 5 second long video segment that displays the TG4 logo.
DevTools only record network traffic after you open the DevTools. So maybe try to open the DevTools, refresh the page, and play the video. You’re looking a link that starts with manifest.prod.boltdns.net/manifest/v1/…/bccenc and ends with manifest.mpd. Here’s the decryption key id and key itself at the time of writing, separated by a colon: