Broody hens regularly raise chicks in those kinds of temperatures. They can handle that kind of heat but if it gets much warmer than 95 they definitely need plenty of water. Too hot is more dangerous than too cool. Make sure your brooder is not in front of a window that lets direct sunlight in to heat them up.
If you provide a heat source of any type, I strongly suggest you make that brooder really big so they can get away from the heat if they need to. Give it lots of ventilation, like keep the top wide open. Some side ventilation would be good too.
I don’t believe in those guidelines either. They are intended to be supersafe. If you follow them the chicks will be absolutely safe even if you have a lousy brooder. But broody hens don’t follow them. The chicks can handle lower temperatures, especially after a very few days. I agree the first couple of days are more critical.
My main suggestion is to watch the chicks and see what they tell you. If you provide a heat source and they crowd the heated area and give a plaintive peep, they are cold. If they get as far from the heat as they can, they are too warm. If they are kind of spread out, they are doing great. I don’t know how cold that garage gets at night, probably not very. You can provide a low wattage heat source at night and see where they sleep, but don’t cook them during the day. You can probably turn the heat off after it warms up just a little bit if you even need the heat at night.
I once raised chicks in my brooder in the coop during a heat wave. Daytime temperatures were over 100, nighttime lows were in the 80’s. My elevated brooder was built out of wire, even the floor. I used a low wattage bulb, can’t remember the exact wattage, and turned that off at day 2 during the day. I turned it off at day 5 for overnight heat. Even in the coolest part of the morning they were lined the far reaches of that 3’ x 6’ brooder. Looking back on it, I could have turned the heat off even sooner, especially during the day, but sometimes you have to experience it to be comfortable.