The XMLNamespaces clause has a default:

XMLNAMESPACES( DEFAULT 'http://tempuri.org/', 

So it's assuming this namespace if none other is provided.

Remove this default and all should be good:

SELECT T.MYCODE 
FROM XMLTABLE(
  XMLNAMESPACES( 
    'http://schemas.xmlsoap.org/soap/envelope/' AS "SOAP-ENV", 
    'http://schemas.xmlsoap.org/soap/encoding/' AS "SOAP-ENC", 
    'http://www.w3.org/2001/XMLSchema'          AS "xsd", 
    'http://www.w3.org/2001/XMLSchema-instance' AS "xsi", 
    'http://scxgxtt.phx-dc.dhl.com/euExpressRateBook/RateMsgResponse' AS "rateresp"
  ), 
  '/SOAP-ENV:Envelope/SOAP-ENV:Body/rateresp:RateResponse/Provider' 
  PASSING  xmltype('<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <SOAP-ENV:Header />
  <SOAP-ENV:Body>
    <rateresp:RateResponse xmlns:rateresp="http://scxgxtt.phx-dc.dhl.com/euExpressRateBook/RateMsgResponse">
      <Provider code="DHL">
        <Notification code="0">
          <Message />
        </Notification>
        <Service type="N">
          <TotalNet>
            <Currency>EUR</Currency>
            <Amount>7.34</Amount>
          </TotalNet>
          <Charges>
            <Currency>EUR</Currency>
            <Charge>
              <ChargeType>EXPRESS DOMESTIC</ChargeType>
              <ChargeAmount>6.46</ChargeAmount>
            </Charge>
            <Charge>
              <ChargeCode>FF</ChargeCode>
              <ChargeType>FUEL SURCHARGE</ChargeType>
              <ChargeAmount>0.88</ChargeAmount>
            </Charge>
          </Charges>
          <DeliveryTime>2020-01-22T23:59:00</DeliveryTime>
          <CutoffTime>2020-01-21T16:00:00</CutoffTime>
          <NextBusinessDayInd>N</NextBusinessDayInd>
        </Service>
      </Provider>
    </rateresp:RateResponse>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>') 
  COLUMNS MYCODE VARCHAR2(20) PATH '@code'
) t;

MYCODE   
DHL