Split ranch style home. 1/2 of basement is finished, the other half has a two-stall garage. House heat duct runs into garage to provide a heated garage. I called it out as a possible safety issue because the ducting inhibits the firewalls’ ability to keep the fire in the garage as well as possibly allowing CO gases to enter the home. Is this correct?
Correct, the (conditioned or not) house air is not to mix with the garage – the garage needs its own, dedicated, heat source.
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Aside from fire separation and fumes it will also cause negative pressure inside the house (assuming there is no return air duct in the garage?). This is an easy thing to look past when people add little spaces and port a heat duct to it. With no return there is no way for the conditioned air routed into the new room to get back in the system.
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…if they want heat in the garage. LOL!
Simon has it! 
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In fact, neither supply nor return, nor any duct opening is allowed that shares air with the house! You may wish to revisit your source of information:
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Simon is correct and we see this in about 8 out of 10 houses.
Never known anyone to correct it and code guys pass on it daily
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