Why stop there?

Let's make it a $10 trillion coin!

Precisely. If there is really no effect on inflation, as the ending quote claims, then why bother making it so small?

It was originally an arbitrary number picked by the person who came up with the idea. In practice, the only reason to make it a relatively low value rather than a septillion is because the White House would want to be in a position to argue that it was an emergency action to ensure continued compliance with the debt limit, the Impoundment Control Act, and the 14th Amendment. Their position would continue to be that it is Congress' responsibility to approve enough borrowing to fulfill their spending mandates, rather than that the President can unilaterally abolish the debt limit (which would itself be unconstitutional).

Trillions are so mundane, now. Why not a quadrillion?

I think the logic of a trillion dollar coin is that it will prevent the debt ceiling from being a crisis for a couple of election cycles, long enough for Congress to get their act together and remove one leg of the trilemma that they have created, but not so long as to make them not have to deal with it at all.