"We assume you are not stupid" means they are making an assumption of intelligence, whereas "we don't assume you are stupid" means they make no assumption one way or the other.

I wonder which is more appropriate?

My intent was to say, in a positive way, "If you are unwilling to learn how a serial console works, or how OpenSSH public key authentication works, I'll refund your money and send you to Linode."

That was exactly how I interpreted it when I read it originally.

I love the minimalist page by the way.

It's great design I think. It attracts exactly the kind of people they want as customers.

I see your point. However, I keep reading it as "We don't (have to) assume you are stupid. (We know so)".

spot on +1

As a stupid person, I was offended.

The fact you understand what the page is even about, means that you are not in anyway a stupid person.

Just because one can be smart, doesn't mean one has lost the chance to be stupid.

That would be stretching it. For all you know, I'm plenty stupid in other, arguably more socially relevant problem domains.