I think it returns the current mouseover help string and to get that string the core engine calls back into the DLL with the current mouseover unit or plot or similar.CyInterface.getHelpString() is not a DLL function - it's a core engine function. No idea what it's supposed to do exactly though, given it has no parameters...
So given the description there is probably some bug in CvGameTextMgr::setCombatPlotHelp for Outrigger vs Dragon Ship.