If I startup MacVim in GUI mode with: mvim --remote
The first time, I get a message:
E247: no registered server named "VIM": Send failed. Trying to execute locally
This is expected, as MacVim wasn't running yet.

Now with MacVim (GUI) running, from a terminal I can do: mvim --serverlist
and I see: VIM as a VIM server that is available.

If I do the same thing, but instead of running the GUI MacVim, I run it in a terminal window I get:
E247: no registered server named "VIM": Send failed. Trying to execute locally
Which again is expected. But apparently no server is actually started.
When I do: vim --serverlist
There is no server found, it just exits without printing anything.
If, from that same terminal I try to do: vim --servername VIM --remote <some_file>
I get the same message as above that there is no registered server named "VIM", and
a new vim is started up.

I have verified that 'mvim' and 'vim' are both running MacVim as installed via homebrew.
VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Mar 13 2018 21:01:05)
macOS version
Included patches: 1-1522
Compiled by Homebrew
Huge version with MacVim GUI. Features included (+) or not (-):