Text reads: "at some point in your life you will be boiling fruit, water, sugar, and lemon juice in a pot to make a syrup or jam. The instructions will tell you to simmer for a certain period of time. Your timer will go off and you will look at the pot and go "hm this doesn't look thick enough, maybe I'll let it go for another 10 minutes". This is the devil speaking. It's only so liquid right now because it's at boiling point. It will thicken when it cools down. Learn from the follies of my youth and do not let this happen to you."
Trusting a recipe to give you the exact timing for something with as many uncontrolled variables as home cooking is not actually any less insane than eyeballing it. The real lesson is: learn how to test for doneness.
One way of doing this for jams and jellies is to stick a plate in the freezer before you start and then drizzle a little on the cold plate when you think it might be done to see if it sets.