How (or why) does this bash script snippet work?

[Dean S. Messing]

In a system script I find this snippet

  nodevs=$(< /proc/filesystems fgrep rootfs)

and don't understand the syntax.  According to the bash man page,

$(< file) is shorthand for $(cat file).  But then the above should read

  nodevs=$(< /proc/filesystems | fgrep rootfs)

However, the latter leaves $nodevs empty whereas the former puts the
stuff that fgrep processes into nodevs as it should.