There is no contradiction here, but there is also no need to pretend Islam teaches that every religion is equally saving. Islam is universal because its call is for every human being, and because God judges with perfect justice; it is not pluralist in the sense that rejecting God's final message is harmless.
Why the criticism sounds forceful
The critic has a real point against a careless slogan. If by "salvation belongs to all humanity" someone means "all sincere religious paths remain equally valid after the coming of Prophet Muhammad (PBUH)," then the Quran does not teach that.
Indeed, those who disbelieved among the People of the Book and the polytheists will be in the fire of Hell, abiding therein. Those are the worst of creatures.
Quran 98:6
And whoever seeks other than Islam as religion, it will never be accepted from him, and he, in the Hereafter, will be among the losers.
Quran 3:85
So the strongest version of the objection is this: the Quran appears to offer salvation widely in one place, then excludes Jews, Christians, and others in another. If Q 2:62 is read as open-ended religious pluralism, Q 3:85 and Q 98:6 do indeed stand against that reading.
What "Islam" means in the Quran
The key is that "Islam" in the Quran is not a tribal label. It means surrender to the one God as He commands. Before Prophet Muhammad (PBUH), following Moses or Jesus faithfully was Islam in that time. After the final messenger is sent and his message is known, accepting him becomes part of surrendering to God.
Indeed, those who believed, and those who were Jews or Christians or Sabians - whoever believed in Allah and the Last Day and did righteousness - will have their reward with their Lord, and no fear will there be concerning them, nor will they grieve.
Quran 2:62
Classical scholars did not usually read this verse as saying that later rejection of Prophet Muhammad (PBUH) is acceptable. They read it as affirming that God's mercy was never restricted to one race or community: whoever, in his proper time and circumstance, truly believed in God, the Last Day, and acted righteously, is not lost before God.
Who is being called "the worst of creatures"?
Q 98:6 does not say every Jew or Christian as a person, ethnicity, or inherited identity is "the worst of creatures." It says "those who disbelieved among the People of the Book and the polytheists." The phrase matters. The chapter speaks about people after "the clear proof" has come to them. The condemnation is of culpable disbelief: knowingly turning away from God's truth after it has been made clear.
The Quran also specifically rejects doctrines it regards as grave falsehood, such as making God one of three:
They have certainly disbelieved who say, "Allah is the third of three." And there is no god except one God.
Quran 5:73
That is not the same as saying Muslims may declare the eternal fate of every individual non-Muslim they meet. Final judgment belongs to God. Scholars distinguished between the ruling on a belief and the fate of a particular person, which may involve ignorance, confusion, coercion, distorted information, or a message never properly received.
What this proves - and what it does not
The verses prove that Islam is not a blanket pluralism where Judaism, Christianity, polytheism, and Islam are all equally valid after the Quran. They also prove that conscious rejection of God's final revelation is spiritually ruinous.
They do not prove that Islam's salvation is ethnically narrow, nor that God damns people arbitrarily, nor that Muslims can casually label all non-Muslims as "the worst of creatures." Islam's offer is universal: every person is invited to worship the one God, repent, and follow the guidance He sent. Its judgment is also just: God knows exactly what each person knew, intended, received, and rejected.
So the slogan needs precision. Islam offers salvation to all humanity, but not by declaring all beliefs true. It offers salvation by calling all people to the same merciful Lord, and by leaving the hidden realities of each soul to His perfect justice.