From the Official BttF FAQ:
we believe Old Biff DID indeed return to a different future — a "2015-A," which would have transformed around Marty, Doc, Jennifer and Einstein (just as Doc explains how 1985-A would change into 1985 and instantly transform around Jennifer and Einstein). This would happen AFTER Old Biff returned with the DeLorean. For this reason, we made sure that Doc had caught Jennifer and exited the McFly Townhouse before Old Biff returned. Thus, by the time Marty and Doc are carrying Jennifer back to the DeLorean, there COULD be other residents in that townhouse — or perhaps the McFlys still live there.
As can be seen in Part 1, changes to the timeline aren't instant. So Old Biff traveled to a future that was in the process of transforming to the one he'd created. Only later do Doc and Marty notice these changes, by which time it's too late to go back to the future to fix them (Related: turns out Old Biff dies before 2015 in the new timeline, so he disappears shortly after arriving).