The half-life of a substance is not the same as the lasting duration of every metabolite of the substance. These are two different things. The half-life of a substance tells you its rate of decay, and can be used to determine the time frame within which it will be detectable as an integral molecular compound. A metabolite is a compound resulting from the metabolic process and chemical pathway to which the substance(s) is related, in this case the compound "M3", which is C26H35ClO9. It obtains when tbol is subject to Hydroxylation and Glucuronidation. Here's a helpful diagram of the chemical reaction-pathways for tbol and its metabolites:
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Nobody is claiming the half-life of tbol is identical to that of its metabolites. That would be nonsense, because different metabolites have different half lives, and these are different from the base substance. That's the entire point: M3 seems to behave thus unusually. You could ask a chemist why this metabolite in particular would be more resilient to decay.
The correct question is: what other metabolites of banned substances have similar lingering time-frames?