Maybe it depends on the McDonald's franchisee in your area, I never see free fries in the app but it's always "any size fries for $1.29" (which is still massively cheaper than their normal price).
Where I live the one closest always has a BOGO breakfast sandwich while the one closer to my parents (3hrs away) will do a “buy one, get $1 off another”.
It’s crazy how much cheaper things are when using the apps for fast food places. I regularly save $3-5 using some $1-item/BOGO/%-off type of coupon.