@MrsPelligrinoPetrichor
Maybe he got extraordinarily lucky with lack of DNA due to the cock of with leaving the body out for so long?
But we’re not talking about DNA, really — it was still fairly rudimentary in the 1990s, anyway, though they did retest and exhumed her body to test again much later — and certainly not initially, just more basic stuff like blood group, or whether the hair in Sophie’s grip was her own, or short and dark.
You’d have to be extremely confident that you hadn’t left a trace at the scene, as you chased and then battered someone fighting back (judging by the state of her hands), probably in the dark, over rough terrain, when, presumably, you were out of your mind with rage/fear/booze or whatever, to offer samples.
I know if I’d done it, I’d be likely to have left hair everywhere — I seem to shed like a cat.
IB would be likely not to have known for definite whether forensics had turned up anything, as by that point he was the chief suspect and no one was going to be leaking authoritative crime scene info to him.
Which is not to say I necessarily believe he’s innocent, only that there’s nothing concrete to link him to the murder, other than circumstantial stuff like his record of violence towards his partner, his lack of watertight alibi for the probable time window of the crime (and even then, the time of death was never established with any certainty), a lot of arguably drunken/sarky confessions and some scratches no one photographed at the time. The only witness statement putting him close to Sophie the weekend of her death and near the crime scene were later withdrawn.