The point specifically is that the "and" in "try and" conceptually should be "adding a second thing" (what they mean by "coordination"), but isn't doing so in a fully regular way. Specifically, it seems like it should coordinate "try to kill mosquitos" and "[actually] kill mosquitos", but that interpretation isn't fully compatible with how the word "and" normally works.

On the other hand, there does seem to be a nuance in the meaning of "try and kill mosquitos" that makes it not just a dialectical form of "try to kill mosquitos"; there's an implication of expecting success. One might also point out that "try" can be replaced with synonyms in "try to" ("attempt to kill mosquitos"), but not "try and" (*"attempt and kill mosquitos"). So this is a very particular idiom.

The conjunction is "and", as in "try and kill" vs "kill and try".