I certainly would agree that rodent nesting materials are ideal tinder, and in the presence of heat from the exhaust manifold or cat converter, it could ignite and initiate a catastrophic vehicle fire. However, I have a VERY hard time believing, as you do, that a daily driver would be subject to enough nesting materials to match the description Honda reported to you: “animals had worked on it a long time”.
Here’s a comparison. Two months ago I purchased a minivan which had not moved in five years. It had been parked outdoors in a rural area surrounded by high grass and underneath a few very large trees. The nearest building was at least 50 yards away. Rodent eating hawks commonly seen in the trees on that two acre parcel, which is surrounded by similar brushy, wooded, rural multi-acre parcels. So I’m not talking manicured suburban sprawl, rather this is older, dispersed, modest housing that is not routinely cleared of grass, brush, other woody debris. In other words, Prime Rodent Habitat. Yes, the engine room of my new ride was kind of cluttered with more fibrous materials than my daily driver, and I made only a haphazard attempt to clear it out. No fires to date.
Second example: I live on 3+ acres, you have to look hard to see another house from my driveway. I KNOW there are rats here…don’t ask. Deer live here-I see them every day. Raccoons and skunks are common. When I travel, one vehicle sits outdoors in a gravel driveway surrounded by grass, brush, and trees, most of which is never manicured…for a month or more. Two consecutive winters I was away for most of six months. No fires ever on the vehicle which remained parked in the driveway.
I could go on…my previous career had me living in an old farmhouse surrounded by a massive wild meadow outside Yosemite where we had regular rodent problems in the house. Some cars would not be used for several days, a week, a month, and no fires ever in any vehicles over decades of people occupying that house. Later, living in a cabin in a heavily wooded area near that meadow, no fires in cars that rarely moved.
This anecdotal information proves nothing. But based on that kind of context, I would be extremely resistant to accepting that your city based daily driver had rodents importing enough nesting materials to match Honda’s claims. I don’t doubt that rodents may have wandered into your vehicle from time to time, but to accumulate a lot of nesting materials overnight is just not realistic. If they can produce “evidence”, I suggest you consult with a university professor who specializes in rodent behavior, for an opinion. I think Honda’s claim is just not credible. Good luck!