No, this is not true. You can do your CPL skill test on a twin-engine aircraft as long as the FTO has the required approval. You will find that most FTOs that have DA42s in their training fleet have this approval.
Thanks for the clarification, Trim Stab; I did see the comment that:
"The skill test(s) will be conducted by the holder of a Flight Examiner Certificate issued in accordance with Part-FCL, in,
• an appropriate class or type of military aeroplane, suitably equipped for the purpose, which has an EASA civilian equivalent class or type, or
• a civilian aeroplane of the appropriate class following training to the satisfaction of the Head of Training of an ATO, Or
• an appropriate type of civilian aeroplane provided the applicant has completed the Part-FCL requirements for inclusion of that type in a Part-FCL licence except the type rating skill test."
... so I presume that overrides the "BFJT graduates shall use a SE/SP aircraft" line. Presumably it's the "satisfaction of the Head of Training of an ATO" line that's important, as otherwise you'd need to satisfy the third bullet point, negating your Part O credits?
If I now understand that correctly, they perhaps could have written it in a slightly clearer manner...!