I believe that's pretty much the only conclusive way to prove it, and yet I believe that even in this case some people will be left unconvinced, yelling that it's simply someone who got access to Satoshi's coins (perhaps by stealing it from the real Satoshi personally). But still, given that it's Satoshi rather than a newbie we're talking about, I don't think it's likely that Satoshi lost access to their coins. However, since it's hypothetically possible and something that could happen to anyone, there are other ways that wouldn't prove that it's Satoshi but could provide some evidence. That could be, for instance, providing an early draft of the whitepaper with an early date being proven.