"At first, anyone could spend anyone's coins"?
He might be talking about the &quot;1 Return&quot; bug

1 RETURN Bug (July 2010)
In July 2010, Satoshi and fellow developer Gavin Andresen received a tip-off about this bug that would have made it possible to spend anyone else’s Bitcoins. In the early releases of the Bitcoin Core, OP_RETURN was defined as:
case OP_RETURN:

{

    pc = pend;

}

break;         

An unlocking script of OP_1 OP_RETURN would then always result in a 1 on the top of the stack, as OP_RETURN would end the script evaluation without modifying whatever was already on the stack. This was fixed by changing the opcode to return false and end the script.

(my emphasis).

Related

What is the &quot;1 RETURN&quot; bug?