Soon.

That's funny.

I really don't understand the ruling at all. It seems to completely contradict how Quake works.

It's an intent ruling to get around a weird problem created by Batgirl's poorly worded effect. It makes it consistent with other targeting restriction/requirement effects (like Cosmic Entity), which is for the best.

Attacking is a sequence of events. You declare an attack and then determine targets. The stipulation included in Batgirl's trait overrides the "targeting all" portion of Quake, but does not negate the attack itself.

Core Rulebook pg 13:

"Once an attack is declared, the following steps occur in order:
1. Determine targets
2. Calculate attack total
3. Determine hits
4. Calculate damage dealt
5. Damage sequence
6. Knock back sequence
7. Resolve attack"