This isn't a typo, but is part of the syntax used by the TZ variable. (The same format appears in the article itself.)

See `man timezone` on a Linux system[1]. Specifically, see the passage that I've quoted below. Note that this is the third of three different formats that the man page describes that you can use in TZ:

> The second format specifies that the timezone information should be read from a file:

    :[filespec]

> *If the file specification filespec is omitted, or its value cannot be interpreted, then Coordinated Universal Time (UTC) is used. If filespec is given, it specifies another tzfile(5)-format file to read the timezone information from. If filespec does not begin with a '/', the file specification is relative to the system timezone directory. If the colon is omitted each of the above TZ formats will be tried.

[1]: https://linux.die.net/man/3/timezone