Yes, water ingress warranty is 6 years, so you should be covered.
Our van is also 2008 model, last year our dealer found the door to the underbed storage was "soft" on the inside. We had not noticed it, but the van had not been used for a few months so we had had no reason to use the door.
Dealer ordered new door under warranty, which of course takes several weeks, and OH agreed to fit it as there was no time to take the van all the way back to them only a few days before our trip abroad. Like you, we could not, and still cannot, see how water could have got into the door, the framing and insert panel was very well sealed together.
When the parts arrived we found Swift had sent a complete door and the frame that goes into the side of the van, plus the sealant required, and insisted the whole thing should be changed, so it turned out to be a longer job than envisaged, though quite straightforward.
We wondered about rain dripping onto the surrounding frame, but had never found any water actually inside the van, so quite a mystery.