As characters have meaningful interactions, they change and grow.
To use the Lord of the Rings as an example: The Frodo who left the Shire was willing to carry the One Ring. The Frodo who left Mount Doom could never do that. He had changed; something inside of him was injured and would not heal.
But here's the thing - Samwise was a side character, and he changed too. The Samewise who left the Shire could not have fought Shelob. He would have run away, or gone for help, or... something else. He would not have dared to battle against something so monstrous.
But Samwise changed over the course of the story. Running from Ringwraiths, watching Boromir die, having to police Gollum for signs of betrayal, all these forced Samwise to grow.
The Samwise in Mordor was not the same person as the Samwise in the Shire, so when the time came he rescued Frodo from Shelob, showing incredible courage.
Compare this to, say, the Bond girls of the 1970s and 1980s. Those characters do not grow. They exist to be conquered by the hero, nothing more.
So I believe the advice you quote is urging you to write female characters like Samwise - as characters with meaningful arcs, where the grow and change as the story progresses.
@wetcircuit has pointed out that Samwise is kind of subservient to Frodo, so that might not be the best example for how to write a "strong female character."
Fair enough. Let's do Macbeth. Lady Macbeth basically bullies her husband into a coup, so I don't think anyone is going to call her "subservient."
But there's no indication that Lady Macbeth had previously been obsessed with power. Her husband seems shocked that she's so into the idea of murdering their way to the top. So Lady Macbeth goes through a very clear character arc / character growth:
Normal -> Temptation -> Fall -> Regret -> Despair
This arc is, of course, related to the Thane of Cawdor's arc; he's the main character. But her growth is compelling. You could imagine a "Wicked/Wizard of Oz" style reframing, where you re-do the play from her point of view instead of his, and it would be interesting.
Because she already has character growth.