If we're disregarding proper maintenance, almost anything can be biodegradable too. Steel will easily rust into dust in an alarmingly short period of time, but we have 100-story buildings made of un-rusted steel lasting for a century and maybe centuries more. Compared to cars from 5 years ago rusted into oblivion even while they're still being driven.
In this case I don't think it really matters. No one says "my car is fine, it's just rusting not biodegrading". What matters is that the car is disappearing right before your eyes through natural processes that could have been prevented with normal maintenance.
Wood, when properly taken care of, lasts forever. Steel, when properly taken care of, lasts forever. Each of them will degrade without proper maintenance, and oddly enough they both have basically the same maintenance needs. Keep them dry, otherwise oil them occasionally.
