Mark is on the money. That diagram shows the line signal going into a downstream IC. The gain control might still affect the signal but it is probably a dual gang pot and Presonus use a clever gain sharing arrangement to minimize noise and maximize headroom? The company are anyway known for excellent pre amps.
However, this question of "will an attenuated mic pre as a line stage bugger my pre amp sound?" comes up regularly. IMHO not likely. Given a decent mic amp design (and Pres' are such as we know!) the addition of a few resistors is hardly going to change things much, in any case the stage will likely be at minimum gain and hence contribute even less noise etc to the signal. The question is even more marginal if the intended pre amp is one of "colour".
But, all my waffle is probably needless? The FS Project has inserts so the whole mic/line stage can be completely bypassed.