Activity intent-filter tag in manifest not working as expected

I have a an application targeting API >= 22 with a unique, single-top activity configured this way in the manifest:

    <activity
        android:name=".MainActivity"
        android:exported="true"
        android:label="@string/app_name"
        android:launchMode="singleTop"
        android:theme="@style/Theme.MyApplication.NoActionBar" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

Notice the android.intent.action.MAIN string above. I have a cloud (Firebase) messaging service configured this way:

    <service
        android:name=".MyService"
        android:exported="false">
        <intent-filter>
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
        </intent-filter>
    </service>

My typical usage scenario is when my application already is running in the foreground when receiving messages. When an incoming message is received, my service's onMessageReceived method is run, and does the following:

@Override
public void onMessageReceived(@NonNull RemoteMessage remoteMessage) {
    super.onMessageReceived(remoteMessage);
    [...]
    Intent intent = new Intent("com.example.myapplication.myservice");
    intent.putExtra("foo", "bar");

    PendingIntent pendingIntent = PendingIntent.getBroadcast(this, requestCode++, intent, PendingIntent.FLAG_ONE_SHOT);
    showNotification(
              getTitle(),
              getBody(),
              pendingIntent);
}

Notice the "com.example.myapplication.myservice" action string above. I have a fragment monitoring broadcast messages this way:

    IntentFilter filter = new IntentFilter("com.example.myapplication.myservice");
    requireActivity().registerReceiver(new MyBroadcastReceiver(), filter);

The receiver does get called successfully, receiving the message created when onMessageReceived was called.

Is this normal, knowning that the <intent-filter> tag for the activity in my manifest should only let android.intent.action.MAIN intent actions in?

Yes, this is normal. You are confusing 2 things.

The <intent-filter> declaration in the manifest controls what Intent actions are used to launch the Activity.

This has nothing to do with broadcast Intents. Broadcast Intents are used to trigger BroadcastReceivers and these have nothing to do with activities.