He's using a lot of highfalutin technical mumbo-jumbo - I don't know why; to feel special, I suppose - but technically speaking it's a sound theory.
The expansion connector on the bottom of the N64 is precious little other than a duplicate of the cartridge slot; if I had a spare Nintendo 64 and new which direction to plug the board in, I would actually try plugging in a case-less cartridge board into it to see if the N64 will boot off of it. I would be extremely unsurprised if it did; that's pretty much the way the Doctor V64 works, all the small intermediary device does is pass the appropriate PI signals up to the CIC chip on the cart you have plugged in rather than to the expansion (aka cart1) slot.
The strange thing about the guy's post is that it almost seems like he doesn't know what DMA is, what with him talking about how only the 64DD can be accessing the bus when it's reading off of the disc. At any rate, the general sequence of events is:
1) Isolate the code in a cartridge that supports interfacing to the 64DD.
2) Put that code inside of code of your own design that can run standalone.
3) Build or procure a flash cartridge that plugs into the top cartridge slot, containing that standalone program and enough free space to store all or most of one side of a 64DD disc.
4) The aforementioned program should, when run, initialize the disc and then step across each logical block of the disc, either copying or DMAing it into either RDRAM or directly into the flashcart.
5) Plug the 64DD in on the bottom.
6) Put the disc to dump in the 64DD.
7) Turn on the N64.
9) Turn off the N64.
10) Plug the flashcart into a reader and dump out the data.
11) Tweak the program to dump any remaining blocks you couldn't fit on the cart, re-flash, thern go to step 5.
Wash hands, wipe hands on pants. Simple.