My understanding is that it is actually based on the Dart, but the Super Dart. Had it based on the Lance, not only it would have been heavier, but it would have a higher gross vehicle weight like the heavies, allowing it to take more payload.
If there is any doubt, the GVW is quite a conclusive evidence - the E300 has a similar GVW limit as the Super Dart, at 14.4 tons, whist the Lance (inc. SLF) are limited at 18 tons.