There is a specific caster level requirement for making an item, it is to be of the level to cast the spell. Where things get questionable is what level do you need to be to make a +6 belt of giant Strength? They are much clearer about weapon and armor bonuses, but if you read the wondrous items you will notice the caster level is the level necessary for the item to function as described, such as Haste lasts 10 rounds (Boots of Haste/speed), so a 10th level caster. But there are a lot of times that the description/requirements don't seem to account/match up. So your DM "house ruling" that the stated CL is required to craft the item isn't a bad idea, especially for the item to function as described. There are a few wondrous items you can make at 3rd level, such as boots of striding and springing, I believe. Many others you will have to wait until 5th, 8th, or higher to make. But if you wanted to craft boots of speed at 5th level, you could, but the duration would be 5 rounds, not 10.
How would I determine the CL for crafting a Belt of Giant Strength? I would use the armor CL rules for enhancements. CL= enhancement bonus times 2.
So I would say a +4 Belt would require a CL of 8 and a +6 would require a CL of 12. But then you have to ask why can't you make a Belt +8 at 16th level? According to the rules those can't be made until you have Epic Craft Wondrous Items Feat.
So his house rule isn't bad for simplifying things.