Does って in this sentence work as a topic marker or have some other meaning?

Question

For clarity: A guy starts speaking in a rude manner to an upperclassman (unknowingly). so the upperclassman's reply is:

いきなりタメ口って
一応　同中の先輩なんだけど

Translation:

Well, that's not very polite!
I'm actually your senpai.

Answer 1

This って is a colloquial topic marker that is used in place of とは. とは/って used like this expresses the speaker's (negative) surprise, bafflement, etc. The senpai used って because he did not expect タメ口.

Related: Function of a って at the end of a sentence?