It appears that he accepted responsibility but hasn't pleaded to the murder charge because: "The murder charge wasn't put to Wayne Couzens today because the defence team are awaiting medical reports they have commissioned. Originally the plea date was set for July with the provisional trial due in October."
SOURCE
"Couzens has not yet been arraigned on the charge of murder.
Couzens will undergo medical and psychiatric reports ahead of a plea and trial preparation date currently set for July 9.
Couzens’ acceptance is not equivalent to a guilty plea.
If he does plead guilty to murder, then the court will proceed to sentencing either later the same day or at a future date"
SOURCE
So I'd say it looks like he's more than likely going for diminished responsibility but is awaiting psychological reports to come in to back it up.