Can anyone clarify the capitalized text of this line of the help text for Soulless:

"Once they sell their soul to the Keeper of Souls, he grants them immunity to both sanctification and HOLY SYMBOL BASED ATTACKS UPON THE SOULLESS FOLLOWER."

In testing a number of spells from a good-aligned cleric vs a soulless evil knight, spells that wouldn't cast without a holy symbol, they had no problem hitting. Stigmata being the exception, though it specifically states in its own help text that it won't work against soulless characters.

What is a holy symbol based attack if not an attack that requires a holy symbol?

I'm pretty sure it's just worded wrong, and is supposed to say something along the lines of "allows them to cast spells without a holy symbol".

"spells, which would normally require them to use a holy symbol."

Pretty sure Red is right.

Soulless clerics and soulless knights no longer need to wear an (un)holy symbol. They no longer receive increased sanctification damage.

As far as a holy symbol based attack, it is any divine spell. However, I don't see anything about divine spells that reduces damage based on soulless victims.