Plus, I mean, unless you’re using a Threadiverse host as your home instance, how often are you typing its name?

Having a hyphen is RFC-conformant:

RFC 952:

1. A "name" (Net, Host, Gateway, or Domain name) is a text string up
to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
sign (-), and period (.).  Note that periods are only allowed when
they serve to delimit components of "domain style names". (See
RFC-921, "Domain Name System Implementation Schedule", for
background).  No blank or space characters are permitted as part of a
name. No distinction is made between upper and lower case.  The first
character must be an alpha character.  The last character must not be
a minus sign or period.  A host which serves as a GATEWAY should have
"-GATEWAY" or "-GW" as part of its name.  Hosts which do not serve as
Internet gateways should not use "-GATEWAY" and "-GW" as part of
their names. A host which is a TAC should have "-TAC" as the last
part of its host name, if it is a DoD host.  Single character names
or nicknames are not allowed.

RFC 1123:

   The syntax of a legal Internet host name was specified in RFC-952
   [DNS:4].  One aspect of host name syntax is hereby changed: the
   restriction on the first character is relaxed to allow either a
   letter or a digit.  Host software MUST support this more liberal
   syntax.

   Host software MUST handle host names of up to 63 characters and
   SHOULD handle host names of up to 255 characters.