It don't mesh with the bigger picture. If there was an explicit suicide note found, Stan Jones would not be the only one who knew about it. Robert Boutflour would then be making up ways for Jeremy to make Sheila write the note ect ect.
It seems to mesh well with the bigger picture. Palm print on bible couldn't be used against Jeremy - so they told the defence no photos existed of the bible. A blatant lie.
Somebody obviously realised, that there would be no way of proving that Jeremy Bamber coerced Sheila in to writing whichever 'note' DS Stan Jones is referring to. Therefore, the note might have worked against the prosecution of Jeremy for the killings. So the only other option was to dispense with it in some way or form.
If it is something we haven't seen in full already, then it was likely to be within the non-disclosed Kinneally report.
What is available would lead me to conclude that the comment was meant literally.
It shows that, early on, Stan Jones wasn't as suspicious as he is routinely made out to be. Wasn't it Stan Jones who also expressed to Ann Eaton, that she could be aiding in an innocent man being sent to prison.. and Ann Eaton responded 'I can live with that'?
Either that, or he is providing a reason for his superior (DCI Jones) not having been suspicious himself - i.e. as a result of said 'note' being found.
OR he (DS Stan Jones) felt safe enough in the company of Met interviewers to reveal snippets of info that potentially exculpated Jeremy. Didn't he invite some Met officers to his leaving do. Recognition for their support.