The ssh root command that you are using is the correct one, so it looks like the problem isn't in that instruction.
There have been known cases of a computer's internet firewall blocking communication with an Intel Aero drone, in which case the recommended advice would be to disable the firewall, or set up a rule in the firewall that permits the drone to be connected to.
Usually, wi-fi connections to a drone are made using a computer running the Linux operating system (the Ubuntu type of Linux is recommended). So a Mac may be able to detect a wi-fi access point but not know how to talk to the drone via wi-fi.
This page has deep details about connecting to the drone's wi-fi.
As suggested on this page, maybe you need to use the drone's wi-fi password 1234567890 to get past security.
Edit: the page says "On macOS and Linux it's possible to use the link-local name rather than fixed IP. Refer to the documentation of your distro on how to enable it ... ArchLinux's documentation works on several Linux distributions and macOS "
I've been able to ssh and scp to other compute boards before like the raspberry pi using my Macbook so I find it a bit unusual that I am unable to do the same with the Intel Aero.
About the drone's wi-fi password, I believe that 1234567890 is only for connecting to the access point itself since the documentation states that there is no password to ssh to the drone. I don't think I would be able to enter that password anyway since I immediately receive the message "Network is unreachable" as soon as I hit enter on the ssh command instead of being prompted for a password.
I will try the link-local approach that you mentioned and see if that works.
It turns out that the command: ssh firstname.lastname@example.org does work. I first tried to make the connections to my Macbook be "link-local only", but it somehow still worked even when this isn't the case and I didn't have to install avahi. The solution was simply to login to email@example.com instead of using the IP address!
In any case, thanks for the help MartyG!
I had seen the ssh firstname.lastname@example.org command during my initial research into your case, but as I only found one case where it was used and everyone else was using the IP address, I wasn't sure that it would work for you and wanted to offer more well-documented solutions first so that I didn't risk wasting your time. I apologize for not suggesting local-link earlier. At least Mac users who research wi-fi connection problems in future will have this post to refer to
Have a great weekend!