I don't think it's a "rule." I think it's a "you can model this this way, if you want...[unspoken: or do something else]." I don't have the SCAG either, so I don't know.
In the case of sunlight sensitivity, and the fact it's a second generation/.25 drow, I would probably just throw the SS on and leave the half-elven traits as it. The + 1/-1 for a first generation, sure. But this is the next generation, and it's a [genetic] "flaw." So I'd just leave the half-elf traits alone and tack SS on.
But that's just me. I suppose you'd have to clear whatever you wanted to do with your DM.