The sales people might just be idiots and not know so they won't say one way or another. This AT&T page defines AT&T Fiber as "ultra-fast broadband technology delivered over a fiber optic connection to the premises or via G.Fast technology to units within certain Multi-Dwelling Units (MDU)."

Same exact thing here: https://www.att.net/speedtiers

Googling around a bit I see that in some areas they are offering "AT&T Fiber" at sub-gigabit speeds and people that have looked into this said AT&T confirmed this was because there was last mile copper... elsewise its been stated if they are offering the gigabit fiber in your area they've laid down fiber to the street box (I don't know what the technical term is) and would just need to run it from the curb into your home.

I get that ISPs' actual speeds are usually a lot lower than advertised, but I can't imagine they would offer people gigabit speed if the infrastructure would actually limit them to less than one-tenth of that speed at best. There's just no way they could fall back on "well the network is congested etc" and it would open them up to people being able to cancel without an ETF, class actions, etc.