I have looked again and I still read this that you have to buy nero 5.5 serial download for $49 then you will be sent the serial for nero 6 free. This would save you the nero 6 download cost of $59.99 - and this, I believe, is the saving nero are offering.
They advertise on their site that the nero 6 download upgrade is $39.99 - so it wouldn't make sense to advertise nero 5.5 and nero 6 upgrade if they are allowing anyone to circumvent these prices for $19 ? Why advertise the other (more expensive) prices atall if you can just pay $19? It doesn't make sense to me, unless you read it as I put in my first paragraph (ie a saving from $59.99 to $49).
Think about it : you could therefore upgrade from nero 4 to nero 6 for $19. I don't think so.
I looked at the french web sites you posted but these are just other forums and nothing to do with ahead. One site says "En collaboration avec http://nero.2037.org" but this has nothing to do officially with ahead - it's just another nero forum with a moderator whose nickname is neromaster.
The reading is ambiguous and don't get me wrong I'd love a nero 6 upgrade for $19, but I think this is too good to be true unless an official nero source can confirm or clarify it, otherwise it's $19 down the drain.