It's not expressly canon. He ends FE7 becoming the Father of an Orphanage on the Bern/Lycia border, and Lugh and company are orphans from an Orphanage on the Bern/Lycia border, and it was a logical leap to make.
So, I watched the Chapter 3 video again, and I thought of a few questions while I did.
How did you decide which characters to add to the playable roster? Support partners?
How much rewriting can we expect for preexisting supports?
I believe you mentioned Lugh is getting a PRF tome in part so that Erk doesn't become overpowered early on. This is still the case?
Assuming you're still using that mug, is Marcus' worn armor supposed to be based on his armor from FE7?
I like how the dialogue isn't just Roy, Guinevere, and/or Merlinus 95% of the time like it is in vanilla FE6. Yes, I know this isn't actually a question.
In short selection went "Okay I want to make Guinivere playable since she's basically the third lord anyway, Galle playable, because he was always intended to be, It'd be nice to bring in Eliwood and Hector since they were in Trial mode." And basically stopped there. I didn't want Hector to always be recruited, hence making him a secret, Erik just came from "I think I can do something neat with this" as did Nino and Sigune. Erk, despite what people might think, was just me rounding out classes (There's only two mounted mages in the game, Erk brings it to 3, in line with every other class more or less), and Lyn... eh, we got Eliwood and Hector and I think I can do something interesting. So far, I think I have.
You can expect re-writing if the Support doesn't make sense (Since now all supports take place in the HQ, not on the battlefield) or if the support was particularly bland or boring.
Lugh getting a Prf is still the case, but I think we've changed Erk himself from simply starting with a lance to a few ideas, after all, someone once brought up that it was "foolish" that a mage had gone the entire way without magic, so it just made sense to patch up that logic hole.
Marcus mug is based on his FE6 official art, I rarely look to FE7 for things to do in FE6. Making the armor worn down was just a way of adding character to him.
And thank you, we work hard to ensure the dialog is spiced up enough while conveying the meaning. It is a little hard to look at Chapter 3 some times, there are moments where I get too melodramatic/wordy, but those are easy things to edit down.