Does making a Feral Tiefling in AL require SCAG to be your "+1" book?
My gut is leaning towards yes since those are variants on a PHB race, yes, but the variants themselves are only in that one book.
You can't use Xanathar's subclasses without choosing Xanathar's Guide as your "+1" book, even though the base class is in the PHB. It should be the same with races, too.
And, according to the quote from NautArch's answer to the linked question:
The variant or optional rules are available when creating your character:
- Half-Elf and Tiefling Variants (SCAG/ToF)
- Option: Human Languages (SCAG) [This option isn't subject to PHB+1]
Seeing as it was not specified to deny the rule, it stands to reason the rule still applies.