Tesla announced they can do this before they even announced the price of the cybertruck. As part of the buyer agreement, there is a stipulation that the original buyer can’t sell the cybertruck until one year after getting it, and that breaking the agreement could result in the vehicle being remotely deactivated.

Honestly, that’s probably what happened here, it was probably less that Elon personally deactivated it, but that it was sold to an intermediary, who then broke the Tesla agreement by “selling” it within the first year, getting it remotely disabled. Alternatively it may have just been a stolen cybertruck, which probably get it deactivated as well.