Email newsletter not rendered properly

I subscribe to an email newsletter, Level Up with Ruby. The email, when rendered through Apple Mail ( on macOS Monterey (12.2.1) looks good. When I view it on the Newsblur site, the code examples are not displayed. When I view through my RSS client, Reeder, the code examples are not displayed. See screenshots

I have created a [gist with the source from the email] (Level Up Email Source · GitHub) - nb email address replaced for privacy purposes

I am not sure if this is a problem with ConvertKit or with Newsblur (or likely some combination of the two), or a problem with my setup.

Thanks in advance for any and all help.