But don't the Tome of Blood horde rules flip it back to what it was pre-errata?
Yeah, just checked and compared.
The core rules seem to be a copy/paste of the Deathwatch rules - you gain a bonus to hit based on the horde size, and score 1 hit per 2 degrees past the first. The poor wording seems to be that they replaced 'Battle Brother' with 'Chaos Marine', implying that it didn't apply to heretics, then the errata replaced the poor wording with almost as poor wording that could be interpreted that only human heretics receive the bonus and not Legionnaires (great show errata team).
Tome of Blood added the optional rule that heretics are allowed to dodge/parry attacks from the horde with a penalty equal to the horde's size modifier, with the trade-off being that they never get more than +10 from size bonus to hit the horde - can't hit as hard, but are able to defend better. It also suggests that hits from the horde against heretics be given tearing dice instead of damage dice so that heretics aren't demolished by hits that are only midly troubling to Legionnaires, and that a handful of stray ranged hits without damage bonus should be distributed among heretics while the bigger hits are allocated to the Legionnaires.
Tome of Blood seems to help gameplay with hordes a lot, but there's always the problem that people on the forum may be using those rules without saying that they are (they're listed as optional after all), or may be assuming that rules that have been mentioned by other posters are different interpretations of the core rules without knowing the additions from ToB.