Yea you literally quoted from the passive thing from the game. Thing is, the Koreans who made this game, imo, aren't exactly the best at translating there stuff into English. So, after using nightingale since like the beginning of time, I feel like this is how the nightingale passive works.

She has three different types of heals:

1: Regular heal with no flashy animation; Night literally just moves her weapon up and a single target gets healed

2: After #1 occurs, this is where part of her "passive comes in." This is the 2nd heal when beams of green light appear, healing her target for 100% of her attack. As opposed to heal #1 which heals for 200% + 7/15/25% based on chain number. So now you my ask why bother with this passive then? for won't one blocks heal a lot more? Well, the thing with this passive is that nightingale's next 5 auto-attacks are replaced with these healing beams of green light. So in essence, your characters get healed FIVE times, so total 500% attack heal. Keep in mind however that this is still SINGLE TARGET, and now we move on to the third heal, which is the what makes Nightingale stand out from all the other healers.

3: This third heal can only be activated DURING heal #2. Meaning, you have to use a block when Nightingale is using her heal #2. This ability is the beauty of Nightingale. Instead of single target green healing beams, the green healing beams are applied to your ENTIRE PARTY. And this also happens FIVE TIMES. So your entire team is healed 500% of Nightingale's attack power. Very, very powerful. And this Heal #3 can also be re-triggered, you don't have to start from the beginning, using a block to start the single beams and another block to start these triple beams. When Nightingale is using this 3rd heal, just activate another block during this heal's duration and the counter gets reset. Meaning, it will heal ANOTHER FIVE TIMES. This is the main reason why Nightingale is a one-block healer.

Keep in mind that I said one-block user, not SPAMMER. Spamming one blocks on Nightingale is very ineffective, for overlap may happen and cause a decrease in efficiency. Two examples are:

1) Using a heal block and using a heal block immediately after before seeing her heal #2 activate. This will not cause heal #3 to activate, for heal #2 has yet to be activated. The point of Nightingale is to get her heal #3 activated as fast as possible, so using heal blocks immediately right after one another will not proc her 3rd heal, but only her 2nd heal. So basically, wait for the 2nd heals first beam of light to appear before you press your next heal block.

2) When Nightingale is in her 3rd heal state, uses a one block early to reset her counter. This one is kinda hard to explain so I'll just explain the scenario: Nightingale is currently in her 3rd heal state, so everyone is getting healed. You see the first set of triple beams come out, and you press a heal block. This would cause Nightingale to reset her heal counter, meaning, she will heal five more times again, which yes, is something that you want to do, but in this scenario, the heal is reset to early. If this were to occur, you would get a total of SIX #3 heals. However, if you were to wait for Nightingale to use her five #3 heals, and then press a heal block. Nightingale would reset her counter and heal another five times. With that, you would get TEN #3 heals. Its hard to explain; try it out in practice mode and you'll see what i mean.

TL:DR Nightingale is uber powerful, She has three different types of heals: 200% heal, 100% single target 5 heal, and 100% whole party 5 heal. There are some simple block management skills that you need to know when using Nightingale. Also, its not "whoever auto attacks gets healed," but instead, its whenever Nightingale attacks that she would heal her party members.