Everyone agrees Jeremy could not drive there. He might be seen or heard.
Not everyone.
Jeremy might very well have been afraid of being seen or heard and thus opted to use the bike but it is unlikely he would have been seen or heard at that hour so he coudl have used the car and I would have used a car.
Jeremy's neighbors never heard him drive to WHF to meet police though we know for a fact he did. They slept through him starting his car and driving away. Other than police no one saw him make the drive to WHF and police saw him only because he was on the side of the road waiting for them to pass or he would have beat them there and no one would have seen him make the journey.
People do not usually get out of bed or even off the couch to go look at a car passing by on the street.
People do not usually pay attention to passing cars unless they expect a visitor.
People who do see a car usually end up seeing it by simply accident as they happen to be outside or looking out a window by chance.
People conversing loudly is usually what gets people to look out a window and notice oh they are leaving or oh they arrived.
The chance of anyone seeing Jeremy drive away or arrive back home would be minimial and the chance of someone along the path being able to identify his car in the dark would be nil.
The chance of someone he knows being out at that hour to be driving on the same path and thus to encounter and also recognize him would be small as well.
Thus Jeremy could indeed have driven there. The fact he told Julie he planned to use a bike and that he took his mother's bike to use is what suggests he used abike.
But the last minute he could have potentially decided to use a car and to have scrapped the bike plan. The notion he can't have used a car is wrong.
Walking 6 miles is a long way. The longer he is outside, the more chance there is of being seen. Why not ride ?
Riding back from WHF gives him more time to clean up & get himself together. Before phoning the police.
The bike was brought to Jeremy's cottage 3 days before the massacre. Coincidence ?
Walking would be the least chance of being seen because in the dark you could not only hide if you see the lights of a car approaching the chance of anyone recognizing you even if they see you is slim. The time and effort involved in a 6 mile walk is why 1 would not want to do it.
The bike is good because JUST IN CASE a neighbor comes home very late and actually notices your car missing and remembers to tell police that can be damaging. To avoid some unlikely freak occurrence where someone actually notices your car gone you would prefer a bike.
He clearly planned to use the bike but whether he went through with the plan or just decided "ah screw it I'll use the car" we will never know.