Long thread on transition through back of surface mounted electrical box - https://forums.mikeholt.com/threads/transitioning-wire-from-interior-2x4-wall-through-back-of-exterior-surface-mounted-panel.2572247/
It seems bundled NM cables are allowed, via the exception below, which would apply to an indoor subpanel. Solution is using either an Arlington NM844 Connector, which nobody knows about.
Can NM cable be bundled out of an electrical panel? Yes, NM cable is exempt from being fastened to the enclosure. It can be fastened insdie the wall cavity.
So in conclusion: 312.5(C) relates to attempts to bundle cable into a box without securing it:
"312.5(C) Cables. Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure."
It says nothing about the method of securing the cable. Securing with usual knockout cable clamps, although standard, is not a requirement.
The exception to 312 allows the bundling of, specifically, NM cable without being secured to the box.
"Exception: Cables with entirely nonmetallic sheaths shall be permitted ..."
In the case of an outdoor box, NM is not allowed. Therefore, the exception does not apply.
Bundled SER and UF from indoor to the outdoor box is allowed, as long as the cable can be secured within the box. In the specific case, the cable transitions from UF to NM in the first receptacle. So the run to the box is UF. The feeder is SER.