> For example, if the smallest coin unit is one cent, then one cannot use a money unit below one cent. This can be enforced by law because only legal tender can be used.
I think the author misunderstood what "legal tender" means. All it means is this: when there is a conflict about a debt and two parties end up using the legal system to resolve it, then the creditor must accept settlement in legal tender.
For example, if I sue you because we had an agreement for you to deliver some gold to me and you failed to deliver, then I may have to accept settlement in euro or dollar or whatever is recognized as legal tender by the court.
In many jurisdictions, people are generally free to use different means of payments or even refuse legal tender when no debt had been established yet. The concept only comes into play in court.