For the record: the safe solution is to send an attachment.

But I agree with you that it's wrong.

Regarding this company, I'm confident they have an option hidden in the settings to not DWIM HTML snippets.

(They probably think it's more efficient to piss off some experts than to bother millions of amateurs)

This reminds me when I spent days debugging a web app which didn't work for 1% if the users in IE.

After tracking down one of them we found out that he was told to run IE in a compatibility mode with a >10 year old version, because another legacy web app didn't work otherwise, and the maintainer was long lost...

Technology is full of workarounds to fix older workarounds.

Like parsing user agent strings which all claim to be the dominant standard browser from 30 years ago...