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.