Depending on how they said it, it may be correct.The last time I used it the OBS did indeed say split tickets via Croydon were not valid, not that all split tickets aren't allowed.
Point to point Single/Return tickets do indeed require the train to call. If one of the tickets is a Season and/or Travelcard, then there is no need for the train to call.
If anyone is charged under circumstances under which they shouldn't be, then please do let us know. So far, our record of demanding refunds from GTR due to such matters has been 100% successful, though we've actually had very few instances of forum members having experienced this (but many more have been threatened and "let off").