Looking through old Infoworld articles it looks like Phoenix signed up to be an MS-DOS sub-licensee at the same time they started selling their clone BIOS to other PC makers, and Phoenix-branded versions of MS-DOS persisted until at least 4.0. One of the oldest articles about the Phoenix BIOS from 1984 specifically says they were offering OEMs a BIOS-plus-their versions of MS-DOS 2.11 and GW-BASIC package deal.
My guess is, then, that they didn't actually sell PCs but some of their lazier white-box customers didn't bother slapping their own branded manuals together, instead simply redistributing a generic "Phoenix" brand one.