I'm still having a hard time finding anything that specifically states conductors installed raceways must be marked.
IMO, if the issue really is just that the conductors are not marked after the outer cable jacket is removed, then I think we would have to apply the requirement of 310.120 unilaterally instead of possibly cherry picking where we want it to apply. I see that 300.14 gives the minimum length for cables and conductors at outlets, junctions and switch points for splicing and terminating, but still doesn't seem to alleviate the supposed issue of conductor marking once the outer jacket is removed. If this is the case, then how could we splice or terminate MC or NM cables after the outer jacket is removed, whether it's just 6-inches or 6-feet.
300.10 states what types of conductor insulation can be used, but doesn't actually have any requirement to mark these conductors with their insulation type.
NEC 300.15 [F] actually seems to allow the type of MC fitting above to be used.
Arlington also has a similar #8600 fitting that has been in use for several years around here for transitioning NM cables to 1/2" EMT for and is allowed without issue as long as the fitting is accessible after installation.

At least manufacturers are willing to disclose what type of conductor insulation is used for MC, SE and UF cables, but with NM we are still only left to assume that it complies with 310.10.