Spoiler is not a Spectre attack. The root cause for Spoiler is a weakness in the address speculation of Intel’s proprietary implementation of the memory subsystem which directly leaks timing behavior due to physical address conflicts. Existing spectre mitigations would therefore not interfere with Spoiler.
The leakage can be exploited by a limited set of instructions, which is visible in all Intel generations starting from the 1st generation of Intel Core processors, independent of the OS and also works from within virtual machines and sandboxed environments.
The root cause of the issue is that the memory operations execute speculatively and the processor resolves the dependency when the full physical address bits are available. Physical address bits are security sensitive information and if they are available to user space, it elevates the user to perform other micro architectural attacks.
This is a Intel specific vulnerability in their proprietary memory subsystem. So like Meltdown and L1TF, Intel gets hit with yet another speculative vulnerability specific to them.