I don't know where he's getting the measurements.
If you go to the HD site, you'll see the jambs for all their interior doors are a little over 4 1/2".
Maybe he was looking at exterior doors for 2x6 stud walls??

Ask for another salesperson or......visit your local retail door store. I usually get better selections and prices there.

Standard width is determined by the width of a 2x4(3&1/2") plus two sheets of 1/2" sheetrock for a nominal 4&1/2" width.

The biggest complaint about service from HD/Lowe's/etc. is that the clerks have very little knowlege/training in many cases.

I am(now retired) a builder/contractor and have often found this to be true. There are usually some people in the store that can offer real info/assistance. You just have to find them.

"standard" jamb width for a 2x4 wall is 4-9/16".
"standard" jamb width for a 2x6 wall is 6-9/16"

With 5/8" wallboard each side of your 2x jamb, (assuming your metal jamb width is 3-1/2"), leaves you with a width of 3-3/4". You could still use a standard 4-9/16" jamb and fur one side of it out with 1/4" screen mould so that you will have an even plane to fasten your door casing. The screen mould will give you a 4-13/16" width.

"leaves you with a width of 3-3/4"."

Correction, meant to say 4-3/4"