Does the XCOM 2 Collection include the XCOM 2 base game?

No. The XCOM 2 Collection includes XCOM 2: War of the Chosen and four DLC packs: Shen’s Last Gift, Alien Hunters, Anarchy’s Children and the Resistance Warrior Pack.

War of the Chosen is an enhanced version of the original XCOM 2 base game with additional content as well as balance and performance improvements.