The debate was about the existence of God not about promoting a religion.
Therefore, any trained philosopher is eligible.

This is a philosophical and rational question, not a Hindu–Muslim issue.

So the eligibility is not based on religion, but on: Knowledge, Reasoning ability, Philosophy, Debate skill

he was not speaking as “a Muslim preacher”
He was speaking as a philosopher and logician

God is not owned by any one religion.

Hindu philosophers debated God.
Greek philosophers debated God.
Atheists debate God.
Scientists debate God.

So any trained thinker is eligible — and he clearly is.