If the concept that one can attain salvation only if he/she is a Muslim is true, then how can you account for those people who have different beliefs or no belief at all?

From the very beginning we should understand from the Islamic perspective that all of the Prophet's of God were Muslims. We don't consider Islam to be the third of the Abrahimic faith. We believe that it was all one. Jesus was a Muslim, Moses was a Muslim and so were Abraham and Adam.

The religion of God as He states in the Qur'an,

" إِنَّ الدِّينَ عِندَ اللَّهِ الْإِسْلَامُ."

Truly, the religion with Allah is Islam.[1]

Islam is submission to the Will of God. Adam was commanded to submit his will to God by not eating from the tree, which was the essential principle here for the forbiddance. That was the religion of Adam. So, for us, as Muslims, we have no problem.

For people who may have been called Christians but who were true followers of Jesus from the first four centuries; the Unitarians - the Trinitarian concept was forced on the objectors-. Those would be considered from the Muslim perspective as being Muslims. They might have been looked at as hierarchical Christians later on, but from our perspective; Jude, the brother of Jesus, who led Jerusalem church and those who followed him, also known as the Ebionites, their belief was that God was one, Jesus was the Prophet of God and we consider them to mainframe the original teachings of Jesus and they are Muslims.

So we believe that wherever all the Prophets (as Prophet Muhammed (ﷺ) said, "Over 124,000 Prophets were sent to human kind, to every nation and tribe.") went and people followed them, they became Muslims. They have the right to salvation as human beings have who follow the teachings of the Prophets who were sent to them, whose originally teachings was, of course, from God.

Reference:

[1] Surah Ale- Imran, 3:19