Hi @jdo,
Congratulations for taking back your privacy and freedom 
I do not know how much advanced you are regarding the GNU/Linux topic, so let’s give it a try. If you don’t understand something, you can simply ask and we (the community) will be glad to help.
The Debian project
As you may have already read here on these forums, PureOS is based on Debian distribution. The Debian community documents many aspects of the project on their wiki. There is a “DebianOn” project, meant to reference common hardware that may or may not support Debian.
The dedicated page for DebianOn Apple hardware is the following one : https://wiki.debian.org/InstallingDebianOn/Apple
Free Software and the FSF
Purism is very bound to the Free Software movement. Sometimes we also talk about FLOSS, standing for Free Libre Open Source Software. The distinction is more important than it sounds: Free Libre Software means that software is “Free as in Freedom, not as in Free Beer”. Open Source software is not necessarily Libre: some companies offer you to read their code, but not modify and/or redistribute it.
The founder of the Free Software movement, Richard Matthew Stallman, also founded the Free Software Foundation. It is a foundation which defends and promotes FLOSS.
They offer a certification for the “purest” distribution, in freedom of software : the RYF (Respect Your Freedom) certification. Debian is not RYF certified, and Purism wanted that certification. You can see the now outdated Purism Freedom Roadmap here : https://puri.sm/learn/freedom-roadmap/ (ping @jeff or @mladen , it might need an update?)
PureOS, Debian, and the RYF
All in all, Purism wanted a computer that runs 100% FLOSS. They created an OS based on Debian, which is stable, usable, and popular. It stripped it out of all its non-free (dis-)abilities, made some packaging effort, added a bit of Coreboot (a Free equivalent to a BIOS).
Will it run on my Mac?
The hard truth is: most hardware manufacturers do not care at all about Free Software. Apple is not different. The second hard truth is: if a modern machine does not run with proprietary drivers, it is very unlikely that it will run with libre drivers. This does not necessarily apply for (much) older hardware.
It seems from the Debian wiki page that your iMac is not supported by Debian, or some features are missing. It is highly unlikely that PureOS will run on your iMac.
But they said there would be freedom?
Purism has a very firm stance towards software freedom. So much to the point they actually make hardware that supports FLOSS.
Others distributions allow non-free drivers and can be more user-friendly. You should give an eye to Ubuntu and their MactelSupportTeam. This could be a good start towards more Freedom. And you know there is still room for more Freedom here