The rule changed last season to include anyone on the team’s 40-man roster as of August 31st as eligible for the post-season. Formerly, only the 25-man roster on August 31st was eligible, with exceptions made for injury replacements. Now, the full 40-man roster is available to be added, no matter when the player was called up after September 1st.
Here’s a good explanation from Ken Rosenthal:
And here’s a full explanation of the July 31st trade deadline and the postseason roster implications from Brett Taylor at Bleacher Report: