Coroners do not ‘guess’. They use the civil standard called ‘the balance of probabilities’ (aka weighing up the evidence for and against a particular conclusion). If they still cannot reach a conclusion, they will record an open verdict.
A friend of mine lost their partner in a situation which sounded an awful lot like suicide (one detail in particular made it seem more likely that suicide was the case). But an open verdict was returned because there were a few factors that provoked doubt. This was actually a relief for my friend, who is more comfortable knowing that they will never really know why it happened; than knowing that their partner didn’t want to live.
The verdict in this case suggests the coroner had access to evidence that convinced him/her on ‘the balance of probabilities’.