I think his ruling is pretty consistent with what I said - just expanding upon it to a situation I didn't discuss.Dragon fear is one of those things that IF you save you're then immune to it for 24 hours.
So:
If you failed your save but get the benefit of heroism? You're not frightened but if the fear effect outlasts the heroism spell (say the dragon smacks the spellcaster, spellcaster loses concentration and you have not successfully saved), you're frightened again - until you can save.
If heroism is active and you're targeted by the dragon fear? You're immune to the effect and don't have to save again (for 24 hours).
IMO Seems like the best interpretation.
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