A bike is also much more identifiable than a car especially by a person on foot. A car is a car going past. A bike could allow a clear ID, or a description of let's say " male, early twenties brown hair, slim cycling"
It's true people can remember numberplates but theyd have no reason to believe the car had been used to transport a killer on his murderous journey at that time
The Sea Wall is a foot/cycle path. If someone else is out in that area, they will be on the Sea Wall.
If he crosses paths with someone else, he will certainly know.
That is why a bike is useful. He can double back if seen on the way to WHF.
Believe the chance of someone else being on the Sea Wall between midnight & 3am was 1 in a 1000.