Why would he use the phrase, "truthfully and forthrightly affirm"???? He could have simply state the student will need to be able to provide a scientific answer. When he uses the phrase, "truthfully and forthrightly affirm" he is going beyond asking for a simple answer. He is asking for an oath.
Because in biology, evolution is the keystone of understanding. If a student refuses to accept that, then they have missed the fundamental point. A physics undergrad that doesn't accept gravity, instead insisting on angels moving the planets and stars around is unlikely to get a recommendation either.
Mr. Spralding knew what Dr. Dini's standards were and yet he signed up for one of his courses. And since letters of recommendation are completely discretionary, Micah Spralding has no expectation of receiving one regardless.
It was a ego-stroking provocation; "Hey look at me. I'm an oppressed Christian!"