While reading your newest article I noticed you used this in your supfile:
If you use this tag on a 5.2.1 REL machine you won't update anything on the system. 'RELENG_5_2_1_RELEASE' is used when you want to cvsup to that RELEASE from, say, 5.1 or 5.0.
I think you wanted to use this tag to track security updates:
The Handbook explains these and other tags.
5_2 applies to 5.2.1 REL as well as 5.2 REL, incidentally.