FTP is one metric that determines performance. There are many others beyond FTP that are independent of FTP. Fatigue resistance is another one.
“Zone 2” defined as a percentage of FTP works up until fatigue starts to kick in. Depending on your experience/training history, this could be after 12 hours, or after only 60 minutes.
After that, continued performance is more strongly impacted by fatigue resistance. Zone 2 is only “all-day” pace, if you have highly trained fatigue resistance. Most recreational/amateur cyclists are not going to be able to ride z2 all day. The phrase is a malapropism. Otherwise riding 400km rides would be no big deal.
Carbs definitely help. They reduce RPE. And anecdotally, eating more carbs also seems to reduce the “fatigue” signal you get.
BUT this isn’t enough. Otherwise everyone and their dog could just eat 90+ g/h of carbs and then ride 400km with no problems.
The more important factor in my mind is developing fatigue resistance. And you do this by increasing the volume of your riding per week, and by doing more regular long rides.