More accurately, the earliest "C Reference Manual" that I have seen has the date 1974-01-15.

It includes all the new features that have differentiated C from B, most of which have been implemented during the summer of 1973. Therefore, by January 1974 C was complete.

The next significant modifications of the language after that date have occurred only during the ANSI standardization process (which ended in 1989, so perhaps the poster above has meant that Amiga happened between K&R C and ANSI C).

I first learned C on an Amiga, back in 1989 or so (Lattice C compiler, which became SAS/C.) All the books and documentation at the time was K&R style. Around 1991 or so, I remember reading K&R second edition, which was ANSI C.

And until early 2000's even UNIX systems were lacking fully conforming ANSI/ISO C compilers, e.g. aCC on HP-UX 10, let alone non-UNIX platforms.

TRIPOS was developed in 1976-1979, with the 68000 port finished in 1981.

Only the Amiga's disk operating system and command-line shell were from TRIPOS. The underlying "Exec" (microkernel) had been developed at Commodore-Amiga: It was close enough to similar facilities in TRIPOS that it made porting relatively simple.

Commodore-Amiga had originally hired another company to write the DOS, called "CAOS" (Commodore Amiga Operating System), originally intended to have been even more similar to Unix than TRIPOS was. But that company had failed to deliver, and the release date was fast approaching, so the choice of TRIPOS was an emergency solution.

Perhaps because compilers were expensive back then.