I can't see anything obviously wrong with your example. This works
without a Google API (and will also use OpenLayers etc):-
<olmap id="olBoundaries" width="600px" height="400px" zoom="18" lat="-35.274529" lon="149.192625" statusbar="1" toolbar="1" controls="1" poihoverstyle="1" baselyr="Google Sat" kmlfile=":property:boundaries:boundaries.kml" summary="The Property Boundaries">
</olmap>
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boundaries.kml:-
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom">
<Document>
<name>Boundaries.kmz</name>
<StyleMap id="m_ylw-pushpin">
<Pair>
<key>normal</key>
<styleUrl>#s_ylw-pushpin</styleUrl>
</Pair>
<Pair>
<key>highlight</key>
<styleUrl>#s_ylw-pushpin_hl</styleUrl>
</Pair>
</StyleMap>
<Style id="s_ylw-pushpin_hl">
<IconStyle>
<scale>1.3</scale>
<Icon>
<href>http://maps.google.com/mapfiles/kml/pushpin/ylw-pushpin.png</href>
</Icon>
<hotSpot x="20" y="2" xunits="pixels" yunits="pixels"/>
</IconStyle>
</Style>
<Style id="s_ylw-pushpin">
<IconStyle>
<scale>1.1</scale>
<Icon>
<href>http://maps.google.com/mapfiles/kml/pushpin/ylw-pushpin.png</href>
</Icon>
<hotSpot x="20" y="2" xunits="pixels" yunits="pixels"/>
</IconStyle>
</Style>
<Placemark>
<name>Boundaries</name>
<styleUrl>#m_ylw-pushpin</styleUrl>
<LineString>
<tessellate>1</tessellate>
<coordinates>
149.1955376935738,-35.27172161228852,0 149.1955641546183,-35.27172517250273,0 149.1945028977079,-35.27756618408651,0 149.1853620012514,-35.27643529497745,0 149.1855994035119,-35.27546410612612,0 149.1913872981926,-35.27609585974587,0 149.1921264646804,-35.27206722997266,0 149.1918889889491,-35.27171874052605,0 149.1924106842149,-35.27147315330129,0 149.1929328137654,-35.27074808132282,0 149.1941310685353,-35.27155407338707,0 149.1955376935738,-35.27172161228852,0
</coordinates>
</LineString>
</Placemark>
</Document>
</kml>
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