I thought the Daedalus (with the failing ion cascade drive or whatever) that preceded the NX was supposed to be a wholly different ship than the sphere-cylinder here?
Possibly; Dave Stern never specifies that in the books.
However, other ENT books ever since
Last Full Measure have made references to the 2140s existence of
Daedalus class ships, as defined by the
Encyclopedia: ships with names familiar from the Okuda book and with an explicit
Daedalus class identity.
Even Mike Friedman's
Starfleet: Year One, which describes development work on the
Daedalus class after the founding of the Federation, funnily enough uses turns of phrase that never explicitly establish the class as not pre-existing. That is, the book could describe development work on an already existing ship class for the purpose of making her useful for the newly founded UFP Starfleet.
It would be sort of fun to think that the sphere-and-can ship indeed began life as a 2141 Cascade Ion Drive prototype (hence looking like a half-baked test rig), was series-produced to some degree before CID was found to be a dangerous dud, and languished in secondary roles during ENT. In the 2160s, she got a revamp into UFP Starfleet's first dedicated explorer, at a time when most of the fleet consisted of much larger, Romulan War surplus combat vessels. She then survived as Starfleet's early
Oberth equivalent until 2196, and was indeed the class of some of the early ships mentioned in TOS and TNG - the
Essex, perhaps the
Archon, possibly the
Horizon, although none of those really is a very good match.
Incidentally, that history would also be a good match for some older fan speculation: the class history could match that of the
Horizon class from good old
Spaceflight Chronology, just with a name swap. Perhaps the embarrassing loss of
USS Daedalus in the CID experiment prompted Starfleet to name the remaining, CID-less ships after the second keel completed, the
Horizon? And perhaps the original name was rehabilitated later on, but some "historical records" would disagree.
Timo Saloniemi