If you scroll down, it appears that Bitwarden sends BasicAuth passwords in the clear over HTTP. This is a huge security hole, far bigger than the subdomain one.
You can't MITM (via Wireshark and similar tools, or over the network), unless you have a root certificate and override the website's certificate with it (assuming the browser allows root certificates, and Firefox can be configured to not allow custom root certificates). So no, not the same thing at all.
And if you meant that in the context of HTTP-only, well, arguably password managers should warn before autocompleting unsecure forms, and browsers should warn before sending the request.