I'm not sure I grokked this. Say there's an Alice-Bob channel of capacity 4, currently 2-2 so balanced. Then Alice wants to add 1 to her side, provides a 1btc input, splices in by consuming existing channel utxo and creating a new one, now the channel is 3-2 and so total capacity is 5. In this non-coinjoin, simple case, the amount received is explicit on chain right? Only thing not explicit is which of A and B received it?

If we were 'coinjoining' in the sense of multiple splices in the same on chain tx, I can imagine that even without equal amounts, it's possible to create obfuscation of linkages, but right now I'm not immediately seeing how the spliced amounts are not explicit on chain.