Well... not defend, but it's just showing your lack of REAL experience with DirectAdmin. That's applying to a system when hosting for multiple accounts with high traffic. With purpose such as email server, I can tell you a fact that our customers sent out 10K+ messages per day without any trouble in a 512MB RAM VPS with DirectAdmin, again with the load is well under 1. In your quote, cPanel is running on 512MB RAM too, 1GB is applying when hosting many accounts.
I am not sure what kind of hosting control panel you have been using in your 11 years of hosting experience (do I remember correctly?
), I don't blame you, but please do not suggest your customers just based on some text. They are wasting money with that kind of suggestion. If you really want to work with and help customers, test your products first and give them suggestions based on real circumstances.
At the end, I just want to re-emphasize that 512MB RAM VPS can do much more than testing purpose or DNS server, if you know how to manage it.