Then it should be trivial for them to provide the source code.

It's trivial in terms that it will cost them nothing, because it's very likely there are no changes to the kernel, or nothing of value nor commercially-sensitive anyway.

It's not trivial in terms of big company bureaucracy - this request will have to go through so many levels of red tape that they (correctly) decided not complying to random people's requests is more profitable.

I'm sure if you actually sue them then they will comply right away, because at that point paying for some engineer's time to tar up the source tree and send it to you now becomes cheaper than lawyer time.

But their analysis is correct in that nobody will waste time/money suing to get what is effectively a stock kernel they can get from the official source anyway. Which is why these complaints are also a bit stupid - they're not asking for anything of value or using the GPL to advance software freedom by freeing up some valuable code, they're just wasting both theirs and others' time asking for something they can already download directly.