The code in question unconditionally zeroes the architecturally-visible registers on entry to the syscall path. If any mis-speculation happens, it has to happe

The code in question unconditionally zeroes the architecturally-visible registers on entry to the syscall path. If any mis-speculation happens, it has to happen after this point, at which point any user-supplied data left over in the register file isn't accessible to speculated code, because even speculated code can only use architectural names to address registers.