Comment by caf

...but I cannot find a source for that claim.

You can see here that in UNIX v6 that /bin/ls had to implement its own readdir() function that calls fopen() on the directory and then getc() 16 times to read each 16-byte dirent:

https://github.com/yisooan/unix-v6/blob/2c7099ee501923775c4c...

You can also see that in those days, struct dirent contained only the inode and dname.