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Accurate detection of acetylcholinesterase (AChE) activity and its inhibitors is indispensable for the early diagnosis and drug discovery of neurodegenerative diseases. In nanozyme-based assays, the sensing mechanism relies on thiocholine (TCh), the product of AChE-catalyzed acetylthiocholine hydrolysis, which specifically inhibits the peroxidase (POD)-like activity of the nanozyme, thereby reducing the oxidation of 10-acetyl-3,7-dihydroxyphenoxazine (ADHP) to the fluorescent and chromogenic product resorufin, enabling indirect quantification of AChE activity. However, conventional nanozyme-based AChE assays are fundamentally limited by a long-standing paradigm conflict: highly active nanozymes typically require acidic conditions for optimal catalysis, whereas AChE and biological systems function exclusively at near-neutral physiological pH. This incompatibility forces tedious pH-switching protocols, which inevitably cause AChE inactivation, signal distortion, and poor reproducibility. Here, we address this incompatibility by engineering an amphiphilic osmium (Os)-ruthenium (Ru) bimetallic polymeric nanozyme (polymer@OsRu) that exhibits robust POD-like activity at neutral pH, achieving high catalytic efficiency without compromising biocompatibility. This work delivers two key advances: (i) materials innovation-the polymer and the Os-Ru bimetallic synergy enable robust catalytic activity under physiological conditions, overcoming the limitations of conventional nanozymes; (ii) platform innovation-the integration of dual-mode (colorimetric and fluorescent) readouts provides intrinsic cross-validation, significantly enhancing detection reliability. The platform achieves ultrasensitive acetylcholinesterase (AChE) detection (LOD = 0.0297 mU mL^-1 for colorimetry and 0.0856 mU mL^-1 for fluorometry) and reliable inhibitor analysis (IC₅₀ = 0.716 µM and 0.92 µM for the clinical drug rivastigmine tartrate), outperforming most reported neutral-pH AChE assays. Beyond analytical performance, this study establishes a generalizable design principle for decoupling nanozyme catalytic efficiency from pH constraints, establishing a versatile framework for physiologically adaptive, multimodal biosensing with broad implications in neurodegenerative diagnostics and drug discovery.