Eat it, Apple
If the tar residue had caused the problem, that'd be one thing. But that's not what they said. If you can show that the smoke congealed the dust into an insulating brick that caused the motherboard to overheat- that's user damage. Document it that way, not some wisp of smoke smell that you call biohazard.
And yes, I've repaired smokers PC's- It's not that big of a deal. Want some really disgusting news? That "dust" inside is mostly dead, rotting skin cells. The nicotine is reasonably well behaved compared to the dead skin cells that with a bit of jostling or a spritz of canned air goes all over the shop, to be inhaled by the techs.
Get real. Use a Vac on the dirty machines, do your damned job, and quit whining.