Interesting, I never encountered that - though that also fits the “2.5 decades” timeframe.
It still shows the author of the error message has no idea about networking: even if we assume network classes apply to RfC 1918 addresses (which they don’t) the majority of those addresses are class A or class B networks.
And looking at it the other way round (using “class C” synonymous with "private addresses) doesn’t work - the majority of addresses in class C space are public addresses.