I think you're overstating what the source is saying a bit. Yes, that's literally what the source says, but it feels exemplary and to the extent it's true, it's true because of a lot of hidden assumptions. The source is basically saying "a normal teen in a vacuum gets sleepy at this time" but it points out many ways in which teens are not in any such vacuum and how they should be trying to get into and stay in that vacuum. A real teen doesn't live in that vacuum.

Teens are already staying up past their natural sleepiness time. Sleepiness is not the problem.

The problem with teens and sleep is not just that we expect them to get up early. We also expect them to go to school, do sometimes hours of homework, do as many extra-curricular activities as they can cram onto a CV, have jobs, spend time with family, and they usually want to have fun with friends and date. They're already sleeping as much as they can. (Many of them are also abusing caffeine and other drugs to manage all of it.)

Humans have two mechanisms for sleepiness. The first is a 'time since last sleep' function that builds sleepiness over time. This is delayed in teen. (This mechanism will respond poorly to later start times.) The second is a 'has it gotten dark' function that responds to the daylight. This is also delayed in teens.

Very likely many teens will simply stay up later. They'll have to in order to keep doing all they want and are expected to do.

However, the delayed start is still a good thing because it is much better aligned with the second mechanism of sleepiness, gives them more opportunities for sleep, and will likely reduce their morning grogginess, even if they don't get more sleep overall.