Let's take Gaeroas and Thureophoroi as examples: going by the descriptions, the former are "impromptu skirmishers" and the latter in turn "melee-skirmishers". Sure, there are the words "impromptu" and "melee-" in those, but they are still skirmishers, and unless I misunderstood your justification for the skirmishing ability, the word "skirmisher" should somehow be more important when defining a unit's abilities. In case you aren't following, I'd like to point out that our previous examples were Lucanian Light Infantry and Cappadocian Hillmen; neither of them is named "skirmishers" or "javelinmen", so why should the word "skirmisher" (from the description) supersede the terms "Light Infantry" and "Hillmen" (from the actual unit names)?
In case my reasoning isn't convincing yet, let's take this a level deeper. Thorakitai are heavier versions of the "melee-skirmisher" Thureophoroi; should Thorakitai be given the ability to skirmish? Samnite Heavy Infantry are said to use different tactics than Roman Infantry, and also to carry more javelins; are Samnite Heavy Infantry skirmishers? Okay, I agree that these are going quite far from the centre point of the discussion, but technically the justification used for Aichmetai/Zanteush could be used for Thorakitai/Samnitici as well.
But the word "skirmisher" and its level of importance aside. I also have a theory, or shall I say, an assumption. I personally believe that there may have been plans to have more South-Italian unit types in EB originally, possibly another unit of Lucanian origin (of course there are the Bruttians, but I'm not talking about those now). If this was the case, one of the two units could have been the (very) light knife-skirmishers described in the unit text of the current Lucanians, and the other would have been something heavier (possibly spearmen, perhaps something like the Samnites). But then the team had to drop the other Lucanian unit for some reason: there was no unit model space/practical purpose/[insert reason here] for both light skirmishers and medium spearmen, but because both javelins and spears seem to have been viewed as important weapons for the Lucanians, the team decided to combine the two units into the Aichmetai we are talking about now.
I'd like to emphasize that what I said above is just a theory of mine. As I'm not a member of the EB team, I have no actual knowledge about what the team had originally intended (all the dropped ideas and so on). Moreover, I don't know much about historical Lucanians or their armaments and battle tactics. But still, this theory would explain the differences between the "physical" appearance of the in-game unit and its text description. I mean, let's face it: the description of the Lucanians really seems to be written for an entirely different type of unit than what the Lucanians themselves are in their current state.
But wait, what on earth does that wall of text above have to do with the skirmishing ability of the Lucanians? Well, it brings me back to my original points. The first being that if there was supposed to be a "line-holder" Lucanian unit that was then squeezed to be only a part of the current unit, the skirmishing ability would just ruin their line-holding capabilities (the running away behaviour I mentioned in my previous post). And the second point is the one I introduced in this post. The unit is called Lucanian Light Infantry, not Lucanian Skirmishers/Javelinmen. Which gives me, at least, the impression of a generic fighting unit and not a skirmisher (contrary to Akontistai for example, which are actually called Hellenic Skirmishers).