...is that The Father did not choose a side.
He was a terrible Father, actually. Look, I get it, you love your children, but how in the hell do you justify allowing your Son, who was apparently a supercharged darksider, to continuously attack your Daughter?
Capitalization intended, as everyone in that episode gets the 'The' treatment because they're supposed to be Important Things or People. But to him, they were not The Son and The Daughter, they were his children.
This is probably kind of confusing if you haven't seen the Mortis episodes in the Clone Wars animated series. But basically Anakin and Ahsoka find Mortis, a planet with lots of weird Force stuff going on (oversimplification, whatever). There are 3 entities: The Father, The Son, and The Daughter. The situation is that The Son and The Daughter are constantly fighting. They are extremely powerful Force users, and aligned with 'the dark' and 'the light' respectively.
According to The Father, anyway.
He's basically trying to stop them fighting, and can periodically put them in a magical Force 'timeout' in a thinly veiled yin-yang circle thing. He tells Anakin he's the only one who can do this, but he's getting tired, and that Anakin needs to stay and take over or there will be galaxy-sized consequences.
He's not siding with either sibling. This is likely supposed to be a profound statement about the nature of the universe. The rest of the episode goes on to show how stupid he's being, but I digress.
Anakin is naturally skeptical. He and Ahsoka stay overnight. Shit happens. LOTs of potentially canon-breaking shit, like Anakin getting a vision of his future. The Son fucks with everyone and eventually kills Ahsoka and The Daughter (who clearly doesn't want to fight him GEE I WONDER WHO THE AGGRESSOR WAS). Ahsoka is revived. The Son dies. The Father dies.
This is never mentioned again in any meaningful way in the series. It's referenced in Ahsoka's show.
The impact of Mortis, however, has echoed through canon, and has led to some misconceptions about how the Force works that I think The Disney Writers are trying to, well, force.
The problem with the Mortis solution is that The Son is 100% on the Dark Side. I don't care that he cried over his sister. It doesn't erase what he's done. And The Father's been enabling the bullshit for who knows how long.
Consider the obvious metaphor: The Son and Daughter are the Dark and Light. If they are frozen and not doing anything (what The Father did), there is peace. But they both still exist. That binding circle is a symbol of wholeness. It implies that the Dark is necessary.
It implies that the extremely evil asshole Son is necessary. It's saying that evil is necessary, that The Dark Side is necessary.
The Dark Side is still evil, right?
Right?
Because the Dark Side is the quick and easy path to power. It is a positive feedback loop of every painful, toxic, negative emotion and disease of the spirit. Those who use it do not love. They covet. They crave power, at the expense of everything else, of everyone else, because it excludes everything but The Self.
One could say that the Sith loathe attachment. After all, people get in the way. Friends are useful, but potential rivals.
Love brought Anakin back, in the end, thank fuck. Anakin loved his son. He chose a side. But thanks to Mortis and The Father's warped idea of 'balance' we now have an idea of the Force as a clumsy light/dark metaphorical magic system where one cannot exist without the other.
Mortis started it. We saw it again in TLJ: remember the Ach-to(sp?) mural? Fml
I've said it before, and I'll say it again: There's no such thing as The Light Side, there's just The Force and the Dark Side. Do you hear me FILONI?
...
....and yes this is in response to Acolyte Things why do you ask. Stop it, Disney. We don't need to balance good by doing more evil.
EDIT: Even if the entire Mortis arc is All Just a Dream, it still touches on How The Force Works. Disney is digging into almost the exact same imagery Mortis uses. TLJ used the same imagery.