I can see how the average person would have thought that the Seinfeld thing was a bass guitar. Listening to it with critical ears, though, there are a few clear signs that it's a sampled keyboard riff. The bends, for example, are a little too assertive to have been done on a bass, unless it was maybe a tenor bass or strung with extremely light gauge strings. Also, the sample voice has a little too much sustain.
Also, as is the case with a lot of sampled instruments from that era, the various notes sound like different pitches of the exact same sound, rather than notes played on varying parts of a string instrument. On a bass, the F at the 3rd fret on the D string sounds different from the one at the 1st fret on the E in more ways than just being an octave higher, is what I'm getting at, and you can hear that not to be the case in the Seinfeld theme.
That said, you can also tell that the guy who played the line on the keyboard had spent a little time playing bass. The note choices and a lot of the articulation and accents do sound like they would probably sound on a bass. It's a well-crafted keyboard line.
Here's another case study from the same era - Howard Jones' tune "Things Can Only Get Better." The bass on this song wasn't fooling anyone; it was obviously a synth line. But - listen to it, the construction of the bassline, including a few of the fills and ad-libs underneath the keyboard solo at 2:40, sounds, to my ears, exactly like it would have if it had been played on an actual bass: