They denied my water damage claim saying it was "gradual damage" - is this a valid reason?

So I filed a claim for water damage from what I thought was a burst pipe under my kitchen floor. The adjuster came out, poked around for maybe 20 minutes, and now I got a denial letter saying the damage was "gradual and long-term" not sudden. The estimate to fix is $14,000.

Here's the thing though - I had zero signs of water damage before this. No staining, no mold smell, nothing. The pipe must have been slowly leaking in the wall where I couldn't see it. Is "gradual damage" a legitimate exclusion they can use here? I feel like they're just looking for any excuse not to pay.

Has anyone successfully appealed a gradual damage denial? I'm ready to fight this but I don't know where to start.