Depends on the definition of "agnostic" being used.
I think this guy is broadly correct. Most of the people who refer to themselves as agnostics are, in actuality, no such thing. They're simply atheists who don't actually know the definition. In other cases, they're atheists who don't want to be atheists, and so refuse to admit that the definition of "atheist" is simply "one who lacks belief in gods", and keep shouting that it means that you actively deny the existence of all gods all the time everywhere.
You make some good points, but I have to ask, who is to say what the correct definition of the word "atheist" is?
Yes, I personally know the definition of the word "atheist" as it is used here on the JREF forum and in other circles of nonbelievers. But I also know how the word is used by the majority of people in our culture. And it is not the way you are describing.
So how can you say someone who uses the word the way the majority of people use the word "doesn't know" the definition?
Furthermore, as skeptics here have pointed out in other contexts, language is a lot more slippery than we usually realize (e.g. cold reading). And over the years, I have come to accept this with other words as well.
So the last time someone asked me if I was an atheist, I answered "define atheist".
Similarly if some asks me if I believe in god, I respond by asking which god or gods? If they say Jesus or Jehovah or Allah etc or if they say an all loving, all knowing, personal god, I say no. There is clearly no evidence and hence no reason to believe in it. I am an atheist in that sense.
But if they define god as the "first cause" of the universe or as an answer to the question "why there is something rather than nothing", I say I am agnostic to that concept. There might be a first cause of the universe which we could refer to as "god" or there might not. Who is to say?
We are talking about the beginning of space and time here, so the very concept of a first cause seems to me to break down. As of 2011, I don't see this as being a question with a knowable answer. And while there has been a great deal of speculation about this, how can any be tested empirically?
(Of course, I reserve the right to changing my mind about this as new information comes to my attention. I had a discussion with Laurence Krauss on this very subject a couple years ago. )
Now most theists don't like that definition, but I have met some who at least understand what I am and am not saying. And I personally think the precision in language with well laid out definitions is the way to go.