testmain.qml

import QtQuick 2.10
import QtQuick.Controls 2.4
import QtQuick.Window 2.2
import QtQuick.Controls.Material 2.3
import QtQuick.Layouts 1.3

ApplicationWindow {
    id: appWindow
    Material.theme: Material.Dark
    title: qsTr("Test QML")
    visible: true

    width: 640
    height: 480

    function referenceLoaderText(){
        console.log("Loader text:" + myLoader.item.text)
    }

    Rectangle {
        id: myRec
        anchors.fill: parent
        color: 'grey'

        ListView{
            anchors.fill: parent
            model: 10
            delegate: Loader{
                id:myLoader
                source: "MyButton.qml"
                onLoaded: {
                    console.log("Loader loaded")                     
                    referenceLoaderText()
                }
            }

//            Component.onCompleted: {
//            console.log("Referencing loader item: " + myLoader.item.text)
//        }
        }
    }
}

You have 10 Loaders since it is inside ListView. Which of these Loaders do you mean when using the id myLoader?
the delegates load components, and the components have local id (they exist only within the component), what is your objective? What do you want to do with the text? You have 10 Loaders with 10 texts, to which of these texts do you want to obtain?