But is it a drop in? Or does it require fitting?
That's a good question. There's a difference between drop in, fitting, and adjusting.

Fitting being removing material until it fits or functions, adjusting would be tweaking a screw etc. Fitting doesn't necessarily mean difficult, you just need to go slow and not remove more material than necessary.
An drop in aftermarket trigger with no adjustments, it will seldom do much more than change appearance, reach, or feel against the finger. Tolerances must be about what the factory trigger provides in order to, um, "drop in".
The simplest adjustable trigger would be one with overtravel only. It may have a set screw to minimize overtravel. If you back that screw out all the way, the trigger will be a "drop in" and it won't reduce overtravel much, if at all. Adjusting it easy - turn in the screw until the trigger will not release the hammer in SA mode, the back out the screw so it does just release the hammer, and finally back the screw out 1/4 turn more (or follow instructions).
A bit more complex is a trigger with both pre-travel and overtravel adjustment - still easy, take your time and adjust pre-travel so trigger will pick up and cock the hammer in DA, then proceed as above for SA.
Some triggers, as in high end guns, will have additional adjustments for reach and fit etc. Some triggers may use a stop that's adjusted by removing material. Here's an example of a trigger I've modified to add an overtravel stop made of 1/8" black Delrin rod. I drill a .2" deep hole on the backside of the trigger, press in the rod, then adjust the length. One way to do this is to cover the frame with thin tape to protect it, then slip a strip of sand paper between the rod to be shortened and the taped frame - pull back and forth, test, repeat.
A P229 Nickel I bought used a few months ago. Trigger stop is the little black button on the back of the trigger. This is a DA/SA gun and you may notice the trigger is back a bit more than stock, as the pre-travel has been removed with an internal mod that I don't sell. The same gain is had easier with a trigger with that adjustment or "fit" built in, like the ArmoryCraft offering).
Take the time to install (or fit) an adjustable trigger and you will be rewarded with a nicer trigger feel.