It seems he realized that the lifetime license did not work out for him as intended, and he started looking for a way out. Possibly there are more users on those lifetime licenses than those on newer licenses, or maybe just enough to, in his mind, warrant this step.
He obviously does not want to abandon the project.
He might have no other way of communicating with some/enough/many/all of his lifetime users, so he chooses to use the one thing he has---the (re-)registration process. Or maybe this is the way, in his mind, to get their attention. Maybe because of GDPR.
He seems to reason that after all this time, asking for another contribution is only fair (even though it goes against the original license). He does offer lifetime users the option to email him and get around the need to pay again.
I can understand both sides, actually.
While in principle I disagree strongly with one-sided changes to agreements such as this, at the end of the day we are all only human, and we have to somehow find a way to make life work for us. It has to be sustainable and worthwhile. I see Khaled trying his best, even though I would probably have recommended a different course of action.
This is not Big Corp effing us over. (Which, sadly, happens a lot, and has much more serious consequences.) This is a single developer with, I believe, no ill intentions, possibly making an understandable but questionable move.
The author of the article seems to look more at the principle of the question that this specific case illustrates. I agree with the principle. We need to be able to trust agreements we entered into.