It's there under "dot".
NAMEdot - execute commands in the current environmentSYNOPSIS. file
NAME
dot - execute commands in the current environment
SYNOPSIS
. file
[etc.]
. is standardized (POSIX 2004, 2008).
.
source is non-standard and specifically not supported in ash, dash or pdksh (so probably it's not in the original Korn Shell either). I.e., it's a Bashism.
source
ash
dash
pdksh