I'm not sure what you're misunderstanding. What I said sounds the same as what you said they said above. It has to do more with calling then it actually does data. When you see the D it means there isn't a data session active so it shows you have a digital signal for calls basically. When a data session is active the arrows basically just override the D because it's active. You should think of it that the D is still there it's just hidden by the arrows because the data is active at that time.
If you were in the business or any tech business even you'd realize that people that work for these companies wouldn't know things like this unless they were a technological geek. Most employees only know what they're trained on and what the scripts in front of them tell them. They don't get any training on what the D on the phone means and honestly I wouldn't be surprised if a lot of them didn't even know what you were talking about unless they saw it. They're going to try and find something in a manual or something that references it and then they're going to give you their best guess to try and satisfy you and get you off the phone.
I honestly don't even know what else to say. You said something about other phones can prove what it is. I'm not sure what that means. If they have a description it should basically be what I just explained.
EDIT: You can pretty much see what I'm talking about with a little messing around. Make sure you're not connected to Wifi so you can see the 3G and arrows and stuff. Then make sure you aren't doing anything using data. It should turn to the D. Then as soon as you use the data the arrows will return. You could also mess with the settings and see if you can find where to turn off data. I know other Android phones have an option to turn off 3G data, not sure if the Fascinate does or not, I haven't looked. If it does, and you turn off data then you should continuously get the D.