It's not a mistake, but yes, it's Assalamualaikum as the title.
As-Salam means 'salamah', a good wish to the person whom we addressed the salam for. As-Salam is a word uttered by those in jannah, showing that As-Salam is a beautiful word uttered by beautiful people.
Rasulalllah SAW will always try to be the first who utters the Salam for sahabah (sahabah are those who lives and the follower of Muhammad SAW). And if sahabah was the one who uttered it first, Rasulallah SAW will answer it longer. For example:
The wish: Salam.
The Answer: Salam alaikum
The wish: Salam alaikum.
The answer: Assalamualaikum warahmatullah
The wish: Assalamualaikum warahmatullah
The answer: Assalamualaikum warahmatullahi wabarakatuh
And Allah SWT as the MOST GRACIOUS will always have a reward for every good deeds that we do. And to wish AS-Salam is highly recommended and answering the Salam is considered COMPULSORY (WAJIB). However, there are four circumstances that makes it not compulsory to answer the Salam.
1. When the Salam is uttered by a person that we are positively sure a non-muslim.
2. When the Salam is uttered by the Imam during a prayer.
3. When the Salam is made into a joke by irresponsible person.
4. When the Salam is uttered after other greetings.
I was quite startled to discover the last circumstance because that is the practice at large. At current, at the opening of any ceremony, the opening address will be the salution to all the guests or VIPs, and later followed by the Salam. However, theres a hadith that Ustaz recite saying that its no need to answer the Salam if it is utttered after other greetings. But I forgot the hadith. And my source of knowledge about this comes from no other but the program 'Tanyalah Ustaz" and today it was with Ustaz Hassan Din Al-Hafiz.