The Makedonian phalanx -- why such depth?

[Paralus]

Whilst it may be true that Philip learned of Epaminondas' echeloned attacks at Thebes, he does not seem to have used them on this occasion, his first major battle.

Diodorus needs to be read with Frontinus:

Front. Strat. 2.3.2
When Philip, king of the Macedonians, was waging war against the Hyllians (Illyrians), he noticed that the front of the enemy consisted entirely of men picked from the whole army, while their flanks were weaker. Accordingly he placed the stoutest of his own men on the right wing, attacked the enemy's left, and by throwing their whole line into confusion won a complete victory.

It is unlikely, unless echeloned back, that the left stood idle whilst the right attacked.

I'm afraid that the way in which you have condensed Polybius' words here is a little misleading. It may not be the case that Polybius has contradicted himself, because the reference to 'close order', and the reference to 16 deep occur in two separate paragraphs, and after describing close order in the first paragraph, in the second one Polybius has reverted to describing what I believe is the 'convention' among Greek authors to refer to depth in 'normal'/open order, and readers would know that such a formation would 'close up' as described in the manuals, and halve it's depth.

I do not think it misleading at all. Your suggestion might be considered pushing the source a little though...

Polybios is clearly describing a phalanx “closed up for action” in the first paragraph in chapter and verse. He then illustrates this description via the Homeric quote. Following directly on that he makes it abundantly clear that the charge he is describing is by a phalanx 16 deep:

From this we can easily conceive what is the nature and force of a charge by the whole phalanx when it is sixteen deep.

In other words, from "what I have just described, we can see just how scary and effective is the charge of a 16 man phalanx".

Clear as a bell.

Or are you suggesting that Polybios insists the sixteen man pahalnx charges in "open order"?