Don't quote me on this, but, I have also heard of these 1960's-1970's houses doing this and it was fine for at least 50 years. No residue or "grossness" beyond normal was noted by the contractor. Best practice? of course not. But, punching holes through your roof can also create maintenance that can be neglected and result in mold. Attics are far from climate/humidity controlled. My bathroom vents up there, since the 90's or so. No Ill effects noted, though it is pointed at water resistant old shingles from an old roof. A stove exhaust adds grease to the mix, but my attic is fairly well ventillated. Maybe that's why the bathroom is not a problem. I am debating doing the same for the season, beats opening the windows and screen door in the winter every time I want to cook a burger, or the fire alarms go off (because the old ductless POS can't clear the smoke).
All kinds of fear mongering about fires, but there are many, many ways a fire can form during a process(es) known for heating and burning things, made up of all kinds of high-current, flammable accelerants and ignition sources. I have seen zero objective evidence linking it to a fan -- doesn't mean it couldn't happen. check your smoke detectors and fire extinguishers!
I'd say it's a wash for temporary use, but temporary might become permanent once you relieve the pain if you do not meticulously budget your time and tasks....
If you're willing to take the risk, maybe do a 1, 5, 10, 20 year study and determine: did hysteria and fires from all kinds of other causes lead to overreaching building codes being created, and extra revenues for cities, officials, and inspectors.
Out in the country ("real America"), you don't need a permit for anything. They laugh that I had to pull a permit to change a dishwasher, or a water heater. You'd think they'd all be dead talking to the city inspector with squeaky wheels looking for grease..