Personally, I have a hard time believing that the Mexican army would come after a barrel that was 21 1/2" long. It could hardly have posed much of a threat.
While I don't have an opinion regarding the provenance of the piece in question, I can see how the Mexican Army might try to sieze a piece like this because (a) it would be perceived as a low risk operation, (b) the enemy would think twice about having other such items if there were substantial risk of attack for possessing such, and (c) it is a government operation and decisions are not always based on logic. Regarding item (a), organized military units have a tendency to hold non-organized opponents in low regard, often to learn later they were wrong.