The absurdity heuristic classifies highly untypical situations as “absurd”, or impossible. While normally very useful as a form of epistemic hygiene, allowing us to detect nonsense, it suffers from the same problems as the representativeness heuristic.
There are a number of situations in which the absurdity heuristic is wrong. A deep theory has to override the intuitive expectation. Where you don’t expect intuition to construct an adequate model of reality, classifying an idea as impossible may be overconfident. The future is usually “absurd”, although sometimes it’s possible to rigorously infer low bounds on capabilities of the future, proving possible what is intuitively absurd.
The absurdity of an idea is often a context-dependent judgment, primarily grounded in the present knowledge and capabilities. The fact that something might become possible in the future does not invalidate its classification as absurd in the present moment it was judged. The “flaw” isn’t in the initial judgment of present absurdity, but in the potential overconfidence of applying that present judgment universally to all future possibilities, or in failing to update that judgment as knowledge advances.
When used in fact based context dependent statements, assertions of absurdity are objectivly true in that moment but do not negate the potential for new information to dismantle the assertion.
Crucially, asserting that a notion is an absurdity heuristic does not validate speculative credulity.