You tagged it correctly according to the wiki. Probably the maplint checker has no knowledge of this type of feature. ‘Not in map features’ errors are not very serious and is mainly(?) intended to spot typo’s so you may disregard it’s complaints.
It would be a good idea however to enhance the maplint-checker, so that “not in map features” do have a correct meaning and the maplint-checker gets more usefull… Is there anything we can do to help with that?