This ideal time to grow garlic is when the ground is cold enough for garlic not to grow the top but still warm enough to grow the roots so the energy stored in the garlic bulb will not be wasted in growing leafs that might be damaged by freeze during the winter ( this depends on how cold that particular winter temperature will be , some garlic leaves bounce back after the freeze, some don’t. But you will never know how cold it is till you go through the winter). However, for a home gardenr, it really can’t tell the difference in planting time as long as you have some garlic bulbs to harvest because the size of the garlic bulbs depend on other facts too (such as amount of /time of / type of fertilizer, water, spaces, weeds etc. in the spring) in the entire growing season. So the ideal garlic planting time is more useful for those commercial garlic growers who have large enough sample data to be collected and analyzed and can see the overall result
I grow garlic every year and I just follow the common sense. I plant my garlic after the trees drop most of leaves from mid Oct to mid Nov. The temperature is cold enough but the ground is still workable. The most important thing is that I get time to plant it