My understanding from the above is that the Let’s Encrypt logo is a trademark, and the logo can not be used ( for any purposes, internal or external) without express permission, which has not been provided. In fact there has been a general request to remove any Let’s Encrypt logos.
Maybe I’m missing something, the question of the topic is
“Let’s Encrypt - is it copyrighted”
and your question was
“we also would like to mark the Let’s Encrypt certifcates with the Let’s encrypt logo”.
The response from LE was regarding
" a badge containing ISRG trademarks like the Let’s Encrypt logo"
tells me that the logo is trademarked / copyrighted
" we can’t allow people to use our logo like this without our permission. " and “I have to ask that you remove it.”
tells me that permission to use has not been granted. It doesn’t say remove only from “public web pages”, So personally I think that information is relevant