I have their referenced text in front me, and the constitutional clause reads read “Agreement OR Compact,” not regard Treat[ies]” and “Agreement[s]...with a foreign Power” as two distinct categories, as part of their discussion of Article II, Section 10.
There is no constitutional subset to a treaty with a foreign power.
No State shall enter into any Treaty, Alliance, or Confederation ... No State shall, without the Consent of Congress ... enter into any Agreement or Compact with another State, or with a foreign Power ...
The author should have included the reference to "Treaty" in the first paragraph of Section 10. It seems to have been a slip on his part.