Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.
I don't know why Crawford would interpret the text that way, and I hope that if it is addressed in an official Errata, he will reconsider what he said (assuming he said it on Twitter). I think that El Suscriptor is correct.
For one, you do expend a luck point if you roll a luck die because "you can spend one luck point to roll an additional d20" whenever you make an attack, meaning that if you choose to use your luck point and roll, it will be expended. This is indicated by the phrase, "You can choose to spend one of your luck points after you roll the die, but before the outcome is determined," which isn't commenting on when you can choose to expend a luck point in and of itself. Rather, it refers to the time that you may expend your luck point as part of the mechanism that it entails. In other words, expending a luck point necessarily gives you the right to roll a die to give you the de facto advantage.
Two, the text unambiguously states that there is no "superadvantage." This is because die is singular, not plural:
You can choose to spend one of your luck points after you roll the die, but before the outcome is determined.
It's saying, "You can spend a luck point [and therefore roll another die] after your attack roll [of one die]." If there were a "superadvantage," the previous sentence would say "die or dice," but it does not. This means that, as El Suscriptor said, you choose between two rolls. It is like doing the order of operations in a simple math equation. For example, if you look at 2*(5+3), you don't multiply 5 by 2 and then 3 by 2, you need to conclude the addition within the paraphrase. Using that same order of logic, you need to conclude the advantage or disadvantage procedure.* If you roll a 1 and a 20 with disadvantage, you take the 1. That is your die after the roll, and then a luck point can be expended (roll a die) before the outcome is determined (i.e. when the DM says you crit missed or whatever).
When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.
Notice that you must decide which of the two dice must be used. It must be concluded, and it's mutually exclusive of any other rolls or procedures. It is a bracketed section of a math equation.