The bit pattern as returned from stat would be different: that's what makes it an ABI break. If I make a file with an mtime of 1952 and then I update the kernel

The bit pattern as returned from stat would be different: that's what makes it an ABI break. If I make a file with an mtime of 1952 and then I update the kernel and that file has an mtime of 2077 or something, that's the kernel violating the contract it has with userspace, and Linux does not break kernel ABI. It doesn't matter whether you think that such timestamps are "lie[s]": the contract is the contract. You have no idea how someone might be using the system, and you don't get to break uses that are legal within the contract but contrary to your sense of good taste.