"they haven't actually complied with the GPL because they have not provided their modified/derived source."
The letter says that they're sending the code "as far as it is required by the OSS licence conditions". They say that there is vehicle code that they haven't sent. There's no indication as to whether this is modified/derived source so there's no basis for assuming that it is.
There seems to be a widespread assumption that because a program runs on some particular platform it must be a derivative of it. It's perfectly possible to write a program which compiles unchanged and runs on multiple platforms so what's the basis for thinking that it's derivative of one of them?