You are correct--because K2 Appit for SharePoint uses SharePoint Online, it is a cloud-based, and browser independent, application.
Whether you access your SP Online site from a PC or a Mac, your experience will be the same.
That being said, there is one minor bug that has been identified when using Chrome on a Mac--the Wizards used in the creation of Workflows seem to lose the ability to accept keyboard input, due to an issue with Silverlight (which is used in these wizards). Meaning, that if you are trying to use the Chrome browser on a Mac to create workflows, you will notice that on many of the sections of the wizard that require keyboard input, you will not be able to type anything into these fields.
This is a known bug and is currently being addressed. In the meantime, the workaround is to use a browser other than Chrome when on a Mac. Alternatively, you can type into a text editor, and cut/paste into the fields in the wizard that require keyboard input.