Governments often paid Indians a bounty for each scalp they took from the heads of their enemies. Sounds like these fellows were trying to pass-off fake human scalps (taken from horses) in order to collect the bounty.
This would accord with the other shenanigans described by the author - the division of a single human scalp into eleven parts, in order to collect the bounty for eleven scalps rather than just one.
Without seeing the original manuscript, I would hazard to guess that the translation is perhaps a little bit of stretch, but is essentially correct.
What an interesting find. I study scalping--largely in the context of Anglo-Native relations, so I have a little less familiarity with Louisiana, but a few possibilities come to mind.
I am currently reading letters from Spanish Colonial Louisiana from the Archive of the Indies in Seville. I came across this letter (dated 1787) which said: