The statement I made earlier was not "the whole truth", mainly because describing the whole truth about CDFS interaction would require an encyclopaedia.
While it is certainly true that plain CDFS is able to read any logical ISO9660 format no matter how big it is, this does not mean that a specific CDFS/CDFSSoft*/DVD reader combination is able to read such a DVD-ROM containing only a plain ISO9660 format. The same effect could be seen with some CD-ROM drives that didn't operate correctly with CDFS, but worked just fine with CDROMFS, despite working through exactly the same CDFSSoft* driver.
Someone who is able to extract the image file from the Foundation DVD on plain RISC OS 4 might like to try to mount that image file with CDFaker/FakeCD - I would expect to see CDFS reading that DVD image just fine.
Maybe now would be the right time to produce the "CDBurn CD Reader"...