New cabinet from old cabinet here: https://ibb.co/2j3VThp
They did the same with my cabinet a month or two back, but it wasn't for G.fast - it was to enable expansion of FTTC capacity. They did a bit of duct digging, and the nearby FTTC cabinet gained an extension which overhangs on the side.
The cabinet was on waiting list for FTTC capacity, which I could see if I searched for my property by address on the wholesale checker (but not by phone number).
Still, if the engineers told you it was for G.fast, it probably is.
Last I heard they are targeting
5.7m homes for G.fast, by March 2021 - scaled back from
10m homes. That's still nearly a quarter of properties, so lots of cabinets to upgrade.