Eren had been using his berserking knowledge and skills passively in the current timeline.
He first trained Steve. Then he took the dual berserkers under his wings to serve as his henchmen. He had told Rodrick to start things from scratch after finding out how the guy operated as a berserker.
His suggestion had helped Rodrick Renar a lot. The royal scion listened to Eren's words and changed his class midway at the cost of stepping back in his ranking journey.
The reason Eren was a capable berserker despite the odds placed against him in the last timeline was that the berserker class amplified the ranker's powers based on their aggression. The butcher had an insurmountable amount of pent-up aggression in his last timeline, serving as fuel to shape him into a worthy berserker.
