> Early on after 4th diagram, author includes sentence : "Because every transaction appears twice, once positive and once negative"

This isn't quite right, but it is a simplification that works here.

> The previous diagram absolutely does not have positive and negative for each transaction! In fact, there is 5000 going into banking account and 500+5 coming out of it.

Yes, those are two separate transactions. Each arrow is a transaction, and each of the accounts that it connects reflects the transaction (one as a positive, the other as a negative.)

> Nothing in 4th diagram is obviously the negative of that 5k transaction, to me.

The arrow with the $5000 has both the positive and the negative.

Each arrow (each edge of the directed graph) represents a "negative" for the account at the tail and a "positive" for the account at the head.