Description of combinator in the mod and mod thumbnail just above, has this:
Combinator that must be placed near a Radar to count and classify incoming biters.
And that's pretty much the difference.
- Alarm sends true/false signal (which you can set) for "if anything hostile is in range".
- Combinator sends multiple signals with count for each type of biters within range.
That's kinda what other thread was about towards the end - adding simplier "yes/no" combinator for a more common "are there any enemies?" use-case, instead of a more complicated counter.
To go with the car analogy, Alarm is like a red on/off warning light on the old car dashboard, which lights up when any parameter is abnormal according to factory presets.
While Combinator is like 10 round gauges next to it, showing the level (or count) for every such parameter, so that you can decide on how close they are to the limit yourself, or maybe if something is abnormal due to a combination of these.
(on the image above, alarm lights up when e.g. fuel-level < 10%, while Sentinel Alarm in this mod lights up on any-hostiles-count > 0)