That's my understanding too, Sinead. An empty stomach means empty of food and is not good for IBS - although, like most things, it seems to me to be quite variable, both between people and and at different times for the same people.
There is a theory that drinking with meals may dilute the gastric juices, but this still would not cause the stomach to empty and is not specifically an IBS issue. There is ALSO a theory that drinking a little with meals helps to stimulate the gastric juices! So I think people should do what they feel comfortable with. When you think about it, there is quite a lot of difference in water content of different foods, so the stomach must be somewhat adaptable to cope with this. Also I find that liquid passes through me much faster than food taken at the same time - it is obviously taking a quicker route!
It really is important to drink plenty on this diet. I definitely notice a difference when I don't drink enough. Personally I prefer to drink some (room temperature) water about half an hour before a meal and then I don't usually drink for about an hour afterwards, but I get in a big pot of Heather's peppermint tea twice a day, usually mid-morning and mid-afternoon. I used to always eat a small snack with this, but don't often feel the need to now. This suits me - everyone needs to find out what suits them, provided they are drinking enough and always avoiding triggers.
Hope this helps.