there are some early unix tapes floating around, and in those early shells, i'm fairly certain the colon was one of only two special-cased code paths after the command line was parsed. does anyone recall more specifically?
I think colon at the start of a line was used for labels for goto statments.
https://www.in-ulm.de/~mascheck/bourne/PWB/goto.1.html
DESCRIPTION
Goto is allowed only when the Shell is taking commands from
a file. The file is searched from the beginning for a line
beginning with `:' followed by one or more spaces followed
by the label. If such a line is found, the goto command
returns. Since the read pointer in the command file points
to the line after the label, the effect is to cause the
Shell to transfer to the labelled line.