Those instructions do not actually provide the correct test for the version of libvte used on Fedora, since our
gnome-terminal-server is in
/usr/libexec. Instead, I'd suggest
$ rpm -qR gnome-terminal|grep vte
vte291(x86-64) >= 0.46.0
Here, we see that 0.46.0 is greater than the 0.36 your tutorial says is required, so this is not your problem. In fact, check this out:
$ echo $COLORTERM
TrueColor is already enabled out of the box on Fedora 25 Workstation.
$COLORTERM is also
truecolor inside of tmux. In fact, this blog post has a simple test script with which I verified that TrueColor is in fact working both outside and inside tmux with no further configuration.
So, this is down to neovim configuration. To make it work in current versions, you need
set termguicolors in your
~/.config/nvim/init.vim. (In versions before May 2016, set the environment variable
1.) This is documented in the neovim log of "breaking changes".