OP themselves even stated that this was an expert dungeon and the tank had pulled "30 mobs." If you're not aoeing that pack regardless of what job you are, then you are holding the group back likely to the point of wiping the group. Tank and Healer cooldowns are not infinite, your "3 mob" analogy does not apply.
Kicking someone for not knowing their job at level cap is not cancerous. No one is asking for O4S dps in an expert roulette, we're asking for basic effort and fundamental knowledge.