Where did Moshe swear about this?
Chomas Anach: This is wrong. Moshe merely repeated Hashem's words (this is a continuation of Hashem's Shevu'ah, from 1:14)! Also, it says "ba'Yom ha'Hu" (the day after the spies returned), and Moshe said Sefer Devarim only in the 40th year!
How does this honor Hashem? Perhaps it belittles Hashem to say that He swore, i.e. His word would not suffice without a Shevu'ah. (PF)
What is the place where Kalev went?
Radak: It is Chevron. It says (in the singular) "va'Yavo Ad Chevron" (Bamidbar 13:22), for only Kalev went there. The others were afraid due to the giants there (listed in the verse). Even though the Torah does not say explicitly that he will get Chevron, Yehoshua agreed, and it is explicit in Shoftim (4:13) that Moshe commanded so.