The rumor doesn’t surprise me. What proves the rumor is false is any American mercs would have done much better than the lackluster performance displayed by the Ukrainian army. For example the separatist body count would have been a lot higher and the buildings would have been back in the authorities’ hands.
The petty gangsters and drunks recruited by the GRU would have been “handled” quickly. Likewise any Spetsnaz would have been identified early and appropriate action taken. Four hundred mercs would be a formidable force.
I doubt they’d be in the field in East Ukraine, meatloaf. But I would be shocked if we didn’t have quite a few trainers and advisors bringing the Ukrainian Army up to speed and shoring up security for the new Kiev regime.