DRM is applied when you deliver through iTunes Producer (if you select DRM Free) and has nothing to do with your EPUB prior to delivering to the iBooks Store.
Prior to iOS 8.4, the only books that could be read on an iPhone was an EPUB. Back then, book developers would have to create an EPUB version if they wanted to read their book on iPhone, and a more feature rich multi-touch (.ibooks) version. iPads have always been able to read .epub and .ibooks formats.
iOS 8.4.3 was the first version of iOS to allow an iPhone to view .ibooks format.
iBooks Author now has the ability to produce .ibooks (Apple Only) and also .epub formatted books. If you used a multi-touch template, you will have a .ibooks formatted book. If you used an .epub template in iBooks Author, you can publish or export to .epub and that book can be uploaded and read on other platforms or readers that read .epub formatted books.
If you export your .epub from iBooks Author it does not have DRM applied to it. If you want your .epub book created with iBooks Author to be read on other platform, upload the .epub file to the other platforms and download it on your non-Mac, or non-iOS device. You can also share it or download it from your non Apple devices directly. The quality of the way the .epub is rendered on the non Apple devices will depend on how good the reader is.
Non Mac computers don't have access to the iBooks Store anyway.