A detent is exactly that, a position where the slider tends to stop. On physical switches like the selector on a DMM they are usually physical indentations with a ball bearing that allows it to click into place. On software sliders and the like they are usually locations where the slider will snap to even when the mouse is a few pixels away.
As to whether a multiselect would be better, that depends on a lot of things. In this case probably, but the danger comes when 95% of the points are out of a few common values, but the rest are spread out. If there are four common values, but 16 outliers where only a couple of devices use that number of bits it becomes hard to find the common values in the list. I have this problem with digikey, where there are thousands of resistor values, most of which are silly little one-off resistors that I only need in special circumstances, and I keep having to scroll through hundreds of options until I finally get to my simple 4.7k resistor.
Perhaps what we really need is a "remove uncommon values" button like the neat "remove outliers" you have. A simple universal implementation could just remove any entries with, say, less than 3 standard deviations from the most common part counts. With more hands-on customization in the long run standardized values like resistor series could select parts from the standard series (i.e. E6, E12, E24, E48, E96, or E192). Listing the number of components with a specific value and even letting us sort a filter list by number of components with the value would both also make it easier to pick values from a list.
So I guess a list might be better in some or even many cases, but in order to be useful some way of dealing with lots of uncommon values is essential.