The comment was in reference to the fact that if you compare the hardware design John Bass used when building his pre-Plus add-on SCSI board for Macintosh 128k/512ks it's *very* similar to the SCSI hardware of the Plus. (It's 95% accurate to say that only the memory locations differ, and in that only slightly.) For a capable programmer with access to the same developer tools John Bass used it appears it would be very trivial to rewrite his disk driver to work with the Plus' SCSI hardware. And since the Dove SCSI boards were compatible with the Plus hardware by extension a driver written on a 64K ROM-ed Plus would work on a real 128/512k equipped with said board and its original ROMs.
The reason's no one's done it is the *right* way to add a mass storage device to a Macintosh is to upgrade to the 128K ROMs. Apple made them available to everyone essentially for free with along with the very worthwhile double-sided drive upgrade. It's a completely pointless exercise to make SCSI work with 64K ROMs. That doesn't stop some people from wanting it.