The Short Version
This claim is misleading. While it's true that training can be maintained with large volume reductions (as in tapering protocols), those involve *planned* reductions, not unplanned 50% performance drops. An unexpected halving of performance on a specific exercise is treated in exercise science literature as a potential warning sign warranting investigation, load reduction, or rest — not routine continuation. The claim's conditional safeguards (normal fatigue markers, normal similar-exercise performance) add nuance but don't override the fundamental concern that an unexplained 50% drop demands caution, not a blanket recommendation to continue.