They are incorrect. The Wyeast packet will not have enough nutrients to feed the yeast for a full size batch. Some would argue that one packet doesn't even have enough yeast, much less the nutrients. When dealing with a liquid yeast packet, and a 23+ liter batch, I'd suggest building up a nice little starter (about 1 liter size). This isn't absolutely essential if you aerate the must well, and feed it well, but using a starter will make sure the yeast get off to a faster start. You can use more than one packet, but that gets expensive.
Whether you use a starter or not, or even if you use multiple packets, you will still need to provide nutrients to allow the yeast to grow to maximum mass. So you'll be needing some DAP and Fermaid K (or other energizer).
Thanks! I'll take your word over the wine/beer people any day of the week
As for one pack or more, it really doesn't matter. My first batch will set me back ~ $800 anyway, so an extra pack won't do much difference