"how is the IRS going to enforce the tax code against a someone who tumbles their coins"
TL;DR - in the same manner as they enforce the tax code against someone who gets income in under-the-counter cash.
If you obtain large amounts of money, then you (presumably) will want to use it to obtain large amounts of goods and services. You can hide your income, but it's harder to hide your spending.
If you spend it all on food, booze, drugs and minor items, then they don't care about you, since the amounts aren't that large.
If you want to buy mansions, flashy cars, high end jewelry and ownership in companies, then they have evidence of you spending much more money than you have declared income. At that point, it becomes your problem - carefully tumbling your coins simply gives the IRS evidence that instead of treating your situation as "forgot to declare that income" (fines) they can prove that you took explicit steps to hide and disguise that income, which carries a risk of jail for tax evasion.
If you obtain large amounts of money, then you (presumably) will want to use it to obtain large amounts of goods and services. You can hide your income, but it's harder to hide your spending.
If you spend it all on food, booze, drugs and minor items, then they don't care about you, since the amounts aren't that large.
If you want to buy mansions, flashy cars, high end jewelry and ownership in companies, then they have evidence of you spending much more money than you have declared income. At that point, it becomes your problem - carefully tumbling your coins simply gives the IRS evidence that instead of treating your situation as "forgot to declare that income" (fines) they can prove that you took explicit steps to hide and disguise that income, which carries a risk of jail for tax evasion.