A couple of thoughts.
1) As Apple engineers were actively developing the 400K disk behind Steve Jobs back, the ROM likely supports the Sony driver found in a 400K external drive at this late date in the prototyping. Then again, they may have kept the Sony drive development ROMs under lock and key lest Steve accidentally stumble upon it like the ill-fated diagnostic port.
2) Even if the ROM supports the 400K drive, it may not support the production model that shipped, in much the same way the Rev. A ROMs do not support the 2nd generation drive that shipped in late 1984.
3) Assuming the 400K external drive would work, there is no guarantee any version of the available early Mac software would boot this Mac, depending on the ROM version. Likewise for the Twiggy drive. Do we really know anything about it for the Mac? The Mac did almost everything differently. Why would Jobs settle for an off-the-shelf implementation of the Twiggy drive? How would one even load an early version of the Mac software onto a Twiggy disk via the drive in that Mac in any reliable way? Assuming it is possible, considering the tweaks that were made in the 11th hour after the 400K drive had been adopted, would any of it actually work with the earliest Mac software available to us? Seems unlikely.