How do you know Kuga was acting? If you're talking about the conversation after he left the seme, that was just his friends joking around and Kuga getting flustered. And the seme is upset because Kuga broke his promise by forgetting him, meaning that all the suffering he went through was for nothing and he's blaming the uke for it.
And he probably WAS manipulated into forgetting, or maybe had a simple case of amnesia, because he clearly doesn't seem to remember living in an orphanage, let alone the seme. So yes, it WOULD be a pretty good excuse because that doesn't make it his fault for forgetting.
It’s obvious he was manipulated, he didn’t just forget you. Get over yourself ( ̄ε(# ̄)Σ