main panel to subpanel conversion

[am1954]

we have a homeline 200a main breaker panel. an electrician just installed a generator ats, so that formerly main panel is now effectively a subpanel. neutrals and grounds are on separate bars, but both bars appear to be isolated from the enclosure, and connected at their tops by a flat bar (don't know the name of that part) that runs continuously from one to the next behind the main lugs.

in the ats, n and g are bonded, and a 4awg bare copper ground runs to a ground rod.

in the former main panel, a 2/0 ground comes in from the ats and is tapped to it's own lug on the back of the enclosure. the neutral bonding screw has been pulled out. the tiny screw that attached the neutral bar to the flat bar that runs behind the main lugs has also been pulled out. a second #4awg bare copper runs from the 'ground bar' to the same ground rod outside.

i'm trying to understand if this is code compliant without installing an auxiliary ground bar in the former main panel and moving all the grounds over to it. aren't all the existing bars in this panel isolated from the enclosure and connected to each other? if so, doesn't pulling the bonding screw essentially just make them one big neutral bar?