Setting the DisableProperty has no effect in creation time. (CSS-state :disabled is ignored)
While running a program, it is works fine.

See Example:

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Tab;
import javafx.scene.control.TabPane;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class TabTestApp extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        BorderPane pane = new BorderPane();
        
        TabPane tabPane = new TabPane();
        Tab tab1 = new Tab("Tab 1 (disabled)");
        tab1.setDisable(true);
        tabPane.getTabs().add(tab1);
        final Tab tab2 = new Tab("Tab 2");
        tabPane.getTabs().add(tab2);
        pane.setTop(tabPane);
        
        Button btn = new Button("disable/enable Tab 2");
        btn.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                tab2.setDisable(!tab2.isDisable());
            }
        });
        pane.setBottom(btn);
        
        Scene scene = new Scene(pane, 500, 200);

        primaryStage.setScene(scene);
        primaryStage.show();

    }
}