Not quite.
Originally, when an attack specifically did two types of damage (not the "1d10+4 damage + 1d6 cold damage" sort of thing being discussed here, but stuff like "2d10+4 fire and radiant damage"), you had to arrive at the total damage, then divide it amongst the various damage types equally. If the numbers didn't divide equally, the first listed type got the remainder. Which was a mess and a hassle. Especially when you encountered damage that had more than two types.
That got simplified down into all of the damage of an attack with multiple types on it being combined, as you said. But that's still specifically for damage that is "x and y damage", not for things that split "damage + x damage". If the damage is already split like that, there was never a need to figure out how to split it afterwards, so it's not covered by the revision.