We have no idea what evidence the police have, but there will be lots we know nothing about. I am as sure as I can be that if the police believed, on the basis of that evidence, that this was even possibly a "stranger killing", then there would be a huge police campaign to warn women in the Bristol area of the danger.
There has been no such campaign. Can you imagine the outcry if another woman was snatched and killed by the same perpatrator(s)?
I can only conclude, therefore, that the police are strongly convinced, on the basis of the evidence they hold, that the killer was close to Jo, and poses no threat to other women.
I am still convinced that the killer was close to Jo.