Statement is probably based on a misunderstanding. AMC service manager F. Neill wrote the Service and Overhaul Manual for the singles, in which he states that for 1957 onward CS and TCS models:
"The big end is robust in construction, [and] the crankpin is materially and dimensionally identical to [that of] the G50 race model."
Yet, the crankpin is NOT identical to the corresponding crankpin of the CS/TCS engines.
Which G80CS engine was the owner referring to?
Although I have never tried the adoption myself, it's unlikely the G50 crankshaft will fit. Width of the assebled flywheels and their diameter will likely be different. Stroke, drive and timing shafts differ, as well as balancing.
In fact, the only G50 component which can be utilised in a G80CS crankshaft assembly is the conrod. Depending on choice of piston, the shorter conrod likely requires the barrel to be shortened.
G50 crankpins provide no improvement in CS engines. A popular modification is fitment of a BSA Goldstar crankpin, which is said to be stronger than AMC's equivalent part.
You may read Ken deGroome's excellent paper on the subject of adopting G50 engine components.