As surely as in poetry takes poetry as the known reference point, to which mathematics is compared. Mr. Russell assumes that you already know with certainty that these feelings are found in poetry, and he is telling you that the same feelings are also found in mathematics.
Here's a moderately famous example of the same kind of comparison:
I knew as well as I knew anything that the oppressor must be liberated as surely as the oppressed. A man who takes away another man's freedom is a prisoner of hatred; he is locked behind the bars of prejudice and narrow-mindedness. I am not truly free if I am taking away someone else's freedom, just as surely as I am not free when my freedom is taken from me. —Nelson Mandela
In other words, it is already clear that an oppressed person must be liberated and that a person deprived of freedom is not free; Mandela is saying that, in addition, the oppressor must be liberated and one who takes away another's freedom is himself not free.
As surely as the Sun rises is a cliché for saying that something is completely sure to happen. Here is a U.S. Senator using a long form of it in a Congressional debate in 1964:
Before the debate is over, [Senator Thurmond] will be relieved of the misunderstanding that he has in mind, because we are going to pass Title I, as surely as the Sun rises in the East and sets in the West.
In other words, the Senate is sure to pass Title I (some legislation currently under debate), regardless of Senator Thurmond's objections. It's practically a taunt.