What you're missing is that the screws that connect the EGC bus to the panel are not the MBJ. The MBJ connects the neutral to the enclosure.
I'm not saying the screws used to mount the EGC bus are the MBJ. I'm wondering whether or not the panel enclosure itself could serve as the MBJ... because in the instance that there is no wire-type jumper between the separate EGC bus bar and the neutral bus... the panel enclosure would be the ground fault current path between the EGC's and the system neutral.
I'll admit I'm no master, but this to me sounds like objectionable current.
And then there's 250.24(B) which explicitly requires an "unspliced" MBJ used to connect:
(1) the equipment grounding conductor(s) and (2) the service-disconnect enclosure to the (3) grounded conductor...
The use of the word "unspliced" to me says wire... no?