By looking the code, the script injects <plaintext> tag while being loaded. So, all the HTML followed by the script element will lose their meanings, just to be rendered as a plain text.

Due to the approach, I guess the start and end tags don't need to be <!-- and -->. It can be anything. The only reason they chose them I guess is that it relieves confusion, though I still think the insertion of the comments in the regular HTML tags looks a bit weird.