Ahh, yes. That's a misconfiguration on their part; looks like the raw domain, without the www, is configured to point to their mailserver, so emails@dashlane.com will work correctly (poorly done, should've had the A record point to the website and the MX record point to the mailserver), so if you manually type https://dashlane.com, it breaks.

That is badly insecure, because it means they don't have HSTS on dashlane.com, only on www.dashlane.com, meaning I could sslstrip them as you described, and the address bar would read dashlane.com instead of www.dashlane.com.