It isn't true.
The ANO has been amended so that pilots holding part-FCL PPLs with SEP Class Ratings may exercise those privileges on non-EASA aeroplanes meeting the SEP Class definition.
Which means that you may fly, for example, a Chipmunk or Piper TriPacer on a part-FCL PPL with SEP Class Rating.
But after 8 Apr 2014 you may not fly, for example, a PA28 or a Cessna 152 on a legacy UK (non-JAR/non-EASA) PPL with SEP Class Rating - except within the scope of the LAPL(A) and then only until 7 Apr 2015. .