So, I reflected on this quote for a bit and thought of something.
This quote applies to both Twilight and Sombra.
For Sombra, fulfilling his destiny means alienating Radiant Hope and becoming a tyrant, but he did it in an instant. Twilight Sparkle, however, stands in the way of that destiny when he rises again. She's a pony he never expected.
For Twilight, the opposite is true. Sombra is also a pony she never expected to stand in the way of her destiny. She also became an Alicorn Princess as part of her fulfilled destiny (caused in a flash of light, an instance) at the cost of her life as a normal Unicorn.
But yeah, that's pretty much it.
not provided yet