Perhaps. Both are grammatically correct, but with slight change of meaning.
Use of "at" suggests that the garden is viewed as a point location. Whereas using "in" would view the garden as a 2d region. If, for example I am speaking of a garden that is on the other side of town, I view the garden as the endpoint of a journey. In that case the garden is a point, and I would use "at".
In the specific example, "arrival" is a point in time, and so "at" seems appropriate. With another verb, "in" might be better. "Were the flowers being watered while Alice was walking in the garden."
The same argument could be used for other examples on that page. You can use both "at the office" and "in the office" with the same slight difference in meaning.