Several hubbers have written hubs on basically the same things I have written on, in some cases after reading and commenting on my hub, in other cases saying they were unaware of my hub.
It seems to me that if the information is at least 80% different than your hub, giving you credit for any information they are using in their hub (like as in a research paper) and entirely a response to your hub, and if your hub is highlighted as the inspiration for their hub, it would not be a hi-jacking. (Was that clear as mud?) ;)
If it's more than 20% your material, material from your hub that is not available in more than one other source online, and is pretty much just a rewrite of your hub, then I would call it a hi-jacking.
Also, delete the comment and if that makes you feel bad, invite him to resubmit a new comment without the link in it.