Comment by MeImCounting

Thats a good point the knives he was familiar with were probably case hardened! So you would get to mild steel just like you said if you kept removing steel.

Nowadays though, knives are generally homogenous or some other composition like san-mai which will have the good steel in the center all the way through so even if you grind it down to a toothpick most modern knives will still have the good steel on the edge. Modern metallurgy means we can have high hardness, high toughness, high wear resistance and high corrosion resistance all in one composition. See Magnacut and other modern powder metallurgy knife steels: https://knifesteelnerds.com/2021/03/25/cpm-magnacut/

I am sure there are still some case hardened knives but for the most part case hardening is a historical technique. Same goes for the type of laminated construction you describe above.

All told the metaphor makes sense historically and in context but taken in the context of modern knives it makes less sense. This is interesting in and of itself because the meaning of metaphors and sayings can change drastically across time and social context.