Hola John

With aunque we move away a bit about the general explanation on "hypothetical events = subjunctive", e.g. "si yo fuera rica... (if I were rich...) 

With aunque in past tenses the reasons behind the indicative and the subjunctive are different. 

Using the same examples as in the lesson, you can have a situation where there is a bit of conversation between two people and some info has already been said/talked about/established. That is what we see in the first example: 

-La reunión de ayer con los clientes fue un éxito.

-Sí, pero aunque fuera un éxito, tuvimos que ceder demasiado por nuestra parte. 

-The meeting with the clients yesterday was a success.

-Yes, but even though it was a success, we had to compromise too much on our side. 

Here there's been some info they've already shared, the bit saying that the meeting with the clients was successful. 

Then, on the following sentence, the second speaker is "repeating" that already shared info using the imperfect subjunctive: sí, pero aunque fuera un éxito..."

 

On the other hand, the simple past indicative could have been used as follows. Imagine this similar dialogue:

-Ayer finalmente pudimos firmar el contrato con el cliente.

-Sí, menos mal, aunque fue duro de convencer...

-Yesterday we could finally sign the contract with the client.

-Yes, luckily, although he was hard to convince...

 

Here I´m using "aunque fue duro de convencer" (not aunque fuera..) because I am presenting this as a new piece of information. The fact that the client was difficult to convince is something that I am "declaring" for the first time. It is as if I said "although he was hard to convince [didn't you notice?] 

I hope this made the contrast a bit more clear...

Saludos