I believe that was where the larger capacity was incorrectly reported. I double-checked the part numbers for the 2018 and the 2019 radiators, and again confirmed that they are the same, so I believe that the radiators are the same for all model years. The radiator part number for my 2016 is also the same as it is for those two years.The ST supplemental manual for my 2016 states a capacity of 6.1 quarts, or 5.8 L, and the regular owner's manual states the same thing.
I'm pretty certain that if Ford had in fact increased the capacity for the radiator for 2019, that it would have then had a different part number. Maybe one of the parts experts on here can chime in on that.
To the OP, it's easy for me to spend your money, but I would really try to avoid one of those cheap generic radiators on eBay if there's any way that you can do it. It's such a critical component in the car, and if the car overheats, you could ruin your engine. If it was me, I would start saving up for one of the better radiators, like a Mountune or a Koyo. Sometimes a helpful way to catch something on sale is to bookmark it on your computer, and check it frequently to see if it goes on sale. My Chrome browser even has a feature where you can ask it to notify you if a price goes down on an item that you're looking at.