I don't find the split fire to be too much of an issue. If you think of every codex as a separate faction with different rules, it's fine mechanically.
That said, the fluff justification *is* silly in comparison with the fluff from any other chapter. Space Marine Sergeants are all veterans of a hundred campaigns, to inlcude Devastator Sgts, and long fang team leads are no more experienced than that. So while the fluff may "justify" it, the fluff boils down to Long Fangs being "moar awesomer" than other marines.
In the Grim Darkness of the Far Future, there is no Common Sense.