The intention of the -implements option is not to use standard interfaces like cloneable. It's there to allow one to write utility methods that operate on multiple XMLC-generated objects that share a common set of ids. The GolfShop demo actually does this, where there are two pages with nearly identical tables. An interfaces is defined that has the expected methods for accessing the table (getElementXXX). The XMLC generated classes then implement this interface.
Under golfShop.presentation.xmlc.cart:
cart.ContentsTable is the table interface.
cart.ContentsHTML, checkout.ConfirmHTML, and checkout.MainHTML
all implement ContentsTable
cart.ContentsTableFormatter.fillInTable will then fill in the tables in all of these class.