1G looks out of whack in this box. Also 4-wire connection is pointless for high resistance like this.
But insulation and guarding on other side become very important.
Yes it is, do not take the 1.21G as serious
This happens when you have space left on final design and do not want to waste it, so there came 2 more posts and this is the result.
If we assume that the isolation distance of your posts to the metal is app. 2mm and we assume that the Rho of the PVC of the binding posts is app. 1E+13 Ohm-cm and allow 20% max resolution influence
There is ~1mm air gap between outer and inner isolation in parallel with ~1cm height of insulation from nut to conductor, so this is not that bad.
Besides the posts, the leads have major influence and proper way would be using triax cable, connectors and proper measurement system (not affordable).
Nevertheless you are right that this is not the correct way to build precision very high resistance standard, though I would not call the used 1G a very precision one
RCA is for temperature measurement with 10k NTC, guard/shield is blue post.