It's much simpler than that. There are two networks in the car - LS-CAN and MS-CAN. Cheap ELM327 based devices can only access the MS-CAN bus. Some lower priority things, including body control module and likely telematics, are on the LS-CAN bus. The diagnostics MDI adapter can talk on both busses.@EV Engineering - I forget, are you working with a 2020 or an earlier Bolt?
If a 2020, you mentioned you've got official diagnostic tools - by any chance can you sniff the CAN buses on the diagnostic connector when performing diagnostics on one of the telematics units?
It appears in the 2020 service manual that the 2020 firewalling setup implemented with the Serial Data Gateway Module (SDGM) is a bit fancier than 2017-2019 - the OBD connector is wired only to the SDGM. The SM implies that there's a way for diagnostic tools to ask the SDGM to talk with the isolated/dirty buses instead of the clean/nonisolated buses, but obviously not how. The answer to "how" would hopefully be answered by a passive bus sniff of an attempt to talk to the telematics module since that's on the isolated buses.
While it's possible they implemented some advanced firewalling, I would suspect it's more likely that if there is a difference, it's only that they moved something from MS-CAN to LS-CAN to free up bus space on MS-CAN.
What specifically are you looking for? I know that some telematics are accessible from the MS-CAN bus in the 2017 at least.