Phalanx warfare: Closing of the ranks

[Paullus Scipio]

Thracian cavalry corselet is a Tube-and-Yoke one made from 3 mm thick leather, re-inforced with scales ( in this case all over, making it the most expensive kind). This is a well known armour principle c.f. ‘face hardened steel’ used on armoured vehicles. The ‘hardened’ material provides the initial resistance, backed up by the bulk of the defence, here the leather, just like on armoured vehicles with ‘face hardened’ steel armour.

This is metal armour. The leather simply provides a substrate to hold the plates together. This leather is so thin that you can stick a pencil through it with little effort. If you want to twist definitions this wildly then we may as well claim that the Roman segmentata is made of leather because of the strapping holding the plates together.

You may choose to call it 'metal' if you wish ( a 'twisted definition' if ever there was one), but the rest of the world calls it 'composite' in recognition of the fact that - just as I explained - it is a combination of the materials which make up the overall protective effect. It grieves me to have to say that the rest of your post is totally incorrect. Not only can you not stick a pencil through 3mm of leather, one cannot stick a pencil, nor stab a pencil dagger-like, through even 1 mm of leather, as I have just tested on an impromptu basis. ( Yes, I was foolish enough to allow the possibility of your assertion being correct ) All that happens is that the lead breaks. Your comment about 'segmentata' is simply an example of two illogical fallacies, to whit, 'reductio ad absurdum' ( pushing an argument to an absurd conclusion)
AND at the same time a 'false analogy'.

As to the effectiveness of 'thin' leather, 17C European cavalry generally wore 'buff' coats whose thickness varied from 1mm to a maximum of 5mm which was considered a perfectly adequate defence against swords, and no doubt 'spent' pistol bullets too - as I know you are well aware of from postings elsewhere. [It is possible, and even probable that Tube-and-Yoke corselets varied in thickness also. Certainly a 'double breasted' one did]

A 5 mm thick Tube-and-Yoke would therefore also be adequate, especially considering it was combined with a shield, giving what was effectively 'spaced armour' protection. Penetration of both was a rarity, but could happen, as the unfortunate Spartan Leonymus of the 'Ten Thousand' discovered when a large 3 ft arrow from a Carduchi longbow killed him after penetrating both. (Xen Anabasis IV.1.18 and IV.2 )
[Digression: Metal armour offered no better protection, the Arcadian Basias being killed after having been shot clean through the head, so one of these arrows penetrated both sides of his helmet ( the anecdote would be somewhat pointless if he was not wearing one), as well as both sides of the skull. Xenophon recounts these two anecdotes to emphasise the astonishing power, for the time, of Carduchi longbows]