Why Internet download manager doesn't come up in this page?

I noticed that IDM (internet download manager) does not come up in this page when the video starts playing: https://www.w3schools.com/html/html5_video.asp

the code behind this page is a simple html5 video tag:

  <video id="video1" style="width:600px;max-width:100%;" controls>
    <source src="mov_bbb.mp4" type="video/mp4">
    <source src="mov_bbb.ogg" type="video/ogg">
    Your browser does not support HTML5 video.
  </video>

When I use the exact same code with the same video in localhost, IDM starts. I have no special setting or exclusions in IDM. So what is the secret behind this page that IDM is not starting?

Some videos are protected by Copyright and use special protected protocol. It is restricted to save any data recived by this protocol. IDM does not support it for legal reasons.(Reference)

To check that this is the reason why video is not shown, you need to open "Options->General" IDM dialog and press "Edit" near "Customize IDM Download panels in browsers".