That's not true. But you get all the upvotes and I get all the downvotes.

Emphasis below is mine.

https://www.indeed.com/career-advice/pay-salary/on-call-pay

What is on-call pay?

According to the U.S Department of Labor, an employee qualifies for on-call pay if they are required to be in the work premises awaiting substantive work, if and when the work becomes available. Alternatively, an employee is also said to be on-call pay if they are at home waiting for work to come up so that they are engaged. On-call pay also applies to cases where an employee is required not to stray away from their places of work as they wait to be engaged.

Those engineers will be considered on-call unless they are working. It's up to Tesla to compensate them as per the law, or better, to encourage their participation.