The major reason that this is surprising is that Beatty is a big shot in this society. He is a captain of firemen. You would think that a guy who is a captain of firemen would be really into obeying and upholding the laws and values of their society.
Knowing a lot of quotes from obscure bits of literature is what you would expect from a subversive in this society. You would expect Faber to come up with quotes like that. But you wouldn't expect it from a guy who (presumably) believes in the rules of the society. It would be like hearing the leader of an athiest group starting to quote obscure parts of the Bible.
This incident occurs as part of "The Hearth and the Salamander", the first section of the book. Montag goes to burn a woman's secret selection of books and she chooses to be burnt with her collection rather than to live. As she dies, she says:
"Play the man, Master Ridley; we shall this day light such a candle, by God's grace, in England, as I trust shall never be put out."
What is key about this quote is that it comes from Master Ridley, who was burnt alive because of their insistence of spreading the translated Bible in England which was viewed as heresy at the time. The woman, likewise, sets herself ablaze for a similar cause, as the government of the time is set to suppress the spread of literature and to destroy it completely, and she is obviously against this.
What is so surprising is that Beatty knows the quote and its origin. As the leader of a force whose job it is to destroy printed matter, it is somewhat shocking that he knows the content of what he is implacably set on eradicating.
Beatty seems like the person that shouldn't even know a quote like and seems obviously from a book it's kind of ironic