Update comment now that I understand it better.

Tools/Scripts/webkitpy/tool/commands/rebaseline.py

Index: Tools/Scripts/webkitpy/tool/commands/rebaseline.py
diff --git a/Tools/Scripts/webkitpy/tool/commands/rebaseline.py b/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
index d6adf3b1b7e895e72c1e88b9a6a894bd6b66bfc5..19144c8e9fd40402da84a84cb3f7eee5c49cc6b7 100644
--- a/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
+++ b/Tools/Scripts/webkitpy/tool/commands/rebaseline.py
@@ -448,8 +448,9 @@ class AbstractParallelRebaselineCommand(AbstractRebaseliningCommand):
 
         # This is so we remove lines for builders that skip this test, e.g. Android skips most
         # tests and we don't want to leave stray [ Android ] lines in TestExpectations..
-        # FIXME: Is this necessary at all now that _rebaseline_commands includes the builders that
-        # used to be skipped because the result wasn't failing on tip of tree?
+        # This is only necessary for "webkit-patch rebaseline" and for rebaselining expected
+        # failures from garden-o-matic. rebaseline-expectations and auto-rebaseline will always
+        # pass the exact set of ports to rebaseline.
         for port_name in self._tool.port_factory.all_port_names():
             port = self._tool.port_factory.get(port_name)
             generic_expectations = TestExpectations(port, tests=tests, include_overrides=False)