He's mostly correct in that you can mostly only use same parts on each weapon, but some are interchangeable. Example, you can mix and match some components from sniper rifles with semi-auto snipers, or combat rifles with support machine guns etc. In fact, in some cases as to make the weapon look not completely modded, you MUST interchange parts. For example, stocks 2-5 on a support machine gun are not viewable, so you must use stocks from a combat rifle to complete it.
The only thing you aren't restricted to is prefixes and titles...which is why you see so many "shredder shredder" or "anarchy anarchy" because of the spread effect you get when using prefixes and titles like that. Personally, I'm a bit shocked we're not seeing more "matador's matador's" :p
The Gearbox forum post is easy to find. Just search willowtree +borderlands. First result.
Only problem is, they can't tell you how to get the save file from the 360 (or PS3) to the PC for editing. I'm guessing Gearbox doesn't like that (as you probably have to mod your 360 to do it) so they have to be real discreet about it.
--- Dark Reality --- What reality are you from please? Must be dark there or something...