Obviously, code that old couldn't have been originally committed to git. But it is quite common to have old repositories migrated to git and one would expect the original time stamps been maintained after the migration.
Sorry, what do you want to say with that? I was talking in general, why Git repositories might contain commits which precede the creation of Git by a large amount. And I am sure, while no one committed the code in 1972 to any kind of repository, that the code snipped with its date is genuine. At that time Kernighan was working with other legends off the Unix world on Unix and the C language.