Sanity-check for COREnn backlash (#21731)

Marlin/src/inc/SanityCheck.h

@@ -3034,10 +3034,12 @@ static_assert(   _ARR_TEST(3,0) && _ARR_TEST(3,1) && _ARR_TEST(3,2)
                   "BACKLASH_COMPENSATION can only apply to " STRINGIFY(NORMAL_AXIS) " on a MarkForged system.");
   #elif IS_CORE
     constexpr float backlash_arr[] = BACKLASH_DISTANCE_MM;
+    #ifndef CORE_BACKLASH
       static_assert(!backlash_arr[CORE_AXIS_1] && !backlash_arr[CORE_AXIS_2],
                   "BACKLASH_COMPENSATION can only apply to " STRINGIFY(NORMAL_AXIS) " with your CORE system.");
     #endif
   #endif
+#endif
 
 #if ENABLED(GRADIENT_MIX) && MIXING_VIRTUAL_TOOLS < 2
   #error "GRADIENT_MIX requires 2 or more MIXING_VIRTUAL_TOOLS."