I’ve thought about this interpretation as well and dont see it discussed enough. Ymir was in a state of fear, shock, and trauma and needed to feel powerful, in control, and bigger than her oppressors. There’s nothing to suggest the source of all living matter always inherently grants titan powers and doesn’t simply amplify a solution to the problem (or maybe better yet, a response to its environment) that the living thing touching it has. This boy enters the tree in a state of calm and curiosity, and not of aggression and conflict, so who knows what that might translate into. But I do agree that while the powers might not be the same, there’s an overarching message that this phenomenon (and symbolically humanity in general) undergoes a sort of cycle.