I agree, but I think the papers take their terminology direct from the terminology used in the initial hearings. The 'mother' doubtless referred to the boyfriend as a 'stepfather' because it played better with social services to give the false impression of familial stability. Baby P's 'mother' did similar.
Statistically speaking, having a step parent enter the family drastically increases the risk to a child. Children under the age of 5 are 8 times more likely to be killed by a 'stepfather' than a bio dad. There are probably very old evolutionary reasons for this.
If you look at all of the recent horrors (Arthur Hughes, Star Hobson, Logan Mwangi etc etc) the abuse really ratcheted up from the non-genetically linked 'step-parent' and it's usually the non-genetically linked 'step parent' primarily responsible for the death.
I myself am a survivor of child abuse and although I was being hit and pushed around before my 'parent' took up with an abusive alcoholic, it escalated drastically once he was in the picture.
For some proper research on non-biological parents and increased risk of abuse:
Daly and Wilson (1994, 2008) reported that rates of fatal assaults of young children by stepfathers are over 100 times those by genetic fathers, and they explain the difference in evolutionary terms. Their study was replicated by comparing updated homicide data and population data from 3...
pubmed.ncbi.nlm.nih.gov
And in case anyone decides to take offence when they have no reason to, this is not to say that ALL step parents are bad. Obviously, not true, child murder is rare. But statistically where a child is killed, you're likely to find a non bio caregiver in the mix somewhere.
That folktale trope of the 'wicked stepmother'? Like many common embedded themes in folklore and fairytales, it exists for a reason.