I've been searching for what exactly the "in-line" part means but I don't get it. Is it the way the chips are positioned, as in a line? If it is, is there another possible configuration?
The "in-line" is about all the contacts being in a straight line on the edge of the PCB.
The SIMM has only actually one set of contacts: the contacts on either side of the PCB are connected.
You can see that on a photo of the slot; there is only one contact for both sides.
On a DIMM, both sides of the PCB carry one line of different contacts:
source: wikimedia user Project Keil