How is there any evidence to support this assertion?

As far as I've heard there is quite literally not a shred of supporting evidence.

Quite the opposite. If Satoshi is still alive that entity is worth around $10 billion. Incentives are a powerful thing and the fact that Satoshi hasn’t made a transaction in years is strong evidence those keys are lost.

Doesn't sound like evidence. Conjecture at most.

My blockchain might be a little rusty, but is it possible to tell the order in which a coin was mined (vs the total) from the information in the chain?

If so, should be possible to find satoshi coins relatively easily, no?

Bitcoin doesn’t really work like that, a “coin” is just a value tied to an address, so individual coins are never transferred. What you can do is see all the incoming transactions to an address to identify which addresses received the earliest mined coins.

Good point. Is there a way to see when an address was created?

To date the opening of the account?

It isn’t proof but it’s definitely evidence.

There's no way to tell the difference between a purposefully dormant "account" and one that has had the keys lost.