Every Sense ROM I have installed has the Software Update page in the settings, even Custom ROM's where it is non-functional. I think it is more likely that it is a carrier modification.
As you are running Sense 3.0 and Android 2.3.3 an official update to Android 2.3.5 is not a big deal really. Actually, I was not to happy about the last update I got in December 2011, and preferred the previous version. It was more resource hungry so I had to cut down on the number of widgets I used, as keeping my previous configuration resulted in my widgets reloading too often.
If you want an update to Android 2.3.5 maybe I could interest you in Android Revolution HD 6.3.3 or if you want to try something very different, how about Ice Cold Sandwich 8.6.1 (Android 4.0.4)