It's a very likely explanation! I wonder if there could be more reasons, too:
1. Maybe Apple didn't like doing business with HD manufacturers of the time or with Seagate in particular. Being able to say "well, we can make our own hard drives" could have been a convenient hedge against those companies: either it strengthened Apple's bargaining position or (if no deal were possible) assured that the hard drives would come from somewhere. It sounds like an elaborate move, but
this video alleges that EDS did the same kind of thing when making deals with IBM for PCs.
2. Very similar to NIH syndrome but not quite the same: businesses often ship their org chart in various ways. If Apple had a disk division, perhaps it just assumed by inertia+default that it would put them to work --- after al, you shouldn't just have them sitting around doing nothing! Sometimes you can come up with justifications for this that get pretty complicated: "sure, Widgets are pricey, but we're investing in learning how to make disks, so it's worth the cost." Just a theory: someone who was at Apple at the time could say whether it holds water.
