Cross build is defined by the existance of a cross file.

mesonbuild/coredata.py

@@ -636,7 +636,7 @@ class CoreData:
                 if type(oldval) != type(value):
                     self.user_options[name] = value
 
-    def is_cross_build(self):
+    def is_cross_build(self) -> bool:
         return len(self.cross_files) > 0
 
     def strip_build_option_names(self, options):

mesonbuild/environment.py

@@ -611,7 +611,7 @@ class Environment:
         self.first_invocation = True
 
     def is_cross_build(self) -> bool:
-        return not self.machines.matches_build_machine(MachineChoice.HOST)
+        return self.coredata.is_cross_build()
 
     def dump_coredata(self):
         return coredata.save(self.coredata, self.get_build_dir())