That's five single coils, so five sets of magnets. 3 humbuckers = 6 sets of magnets. That's 1 more. Each magnet does have a braking effect on the strings, that's just physics. Maybe it has less overall effect on bass strings than guitar strings, with them having more mass and momentum. Setting the pickups a bit further away from the strings helps to increase sustain, and pickups with weaker magnets will have less effect than more powerful magnets. But add in a compressor and the apparent sustain goes up. Add in more volume and the sustain level goes up. So there's a whole combination of things that can affect the sustain.