I just attempted this too and got the same message as you.
Reading the comments on the blog post where the unlocking form is located, the creator said to ignore it, if you're s-off then you're unlocked.
I have no sim to test at the moment, I'll report back tomorrow.
But I am not unlocked, even though I am S-OFF, via Revolutionary.
I inserted a foreign SIM, it asked me for an unlock code..