bug#80745: fmt --width arbitrary limit?

I ran
  echo in | fmt -w 9999 >/tmp/out
and was surprised to get the error:
  fmt: invalid width: '9999': Numerical result out of range

Why is it fmt's business to decide what value is acceptable?
Does POSIX require this arbitrary limit?
I don't see the limit documented in coreutils.texi or fmt --help.

This is with fmt (GNU coreutils) 8.32. Sorry if it is fixed in a newer
release.

Rationale: I was using fmt as a quick way to concatenate a bunch of
short lines into one. I am aware there are many other ways to do it. -k