They most likely are, but Salvatore contradicts or omits several lore details
First of all, welcome to SE!
As for your question, even determining what is canon for Forgotten Realms is kinda tough.
Ed Greenwood, the creator of the Forgotten Realms, says in regards to "canon":
any published source relating to the Forgotten Realms campaign setting. This ...
These variant subraces are not connected to a specific campaign
Mordenkainen's Tome of Foes is not connected to any specific campaign, and the first few chapters are generic "global" changes (emphasis mine to draw attention to the generic language):
The first five chapters present material for a Dungeon Master to add depth to a campaign that involves the ...