So there needs to be OCPD on the feedthrough conductors where they leave the MDP?
Yeah, I don't know, as I find the text very ambiguous. Is "supply-end" with respect to the grid, or to the interconnected power source, or either? Also, as the last sentence is written in the permissive tone, there's nothing that actually seems to prohibit using (B)(3)(1)through (B)(3)(3) when the predicate of that sentence is not satisfied.
For reference, below is the 2023 First Draft version of the corresponding section, 705.12(B)(5). I find it to be clearer (although it still has the same problem with the permissive tone in the last sentence). What I can't tell is whether what it says is what the CMP thought they were saying in the 2020 NEC, but they messed up the wording the first time, or if this represents an intended substantive change.
Cheers, Wayne
"Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors or are supplied by feed-through conductors. The feed-through conductors shall be sized in accordance with 705.12(A). Where an overcurrent device is installed at either end of the feed-through conductors, panelboard busbars on either side of the feed-through conductors shall be permitted to be sized in accordance with (1) through (3)."