True. But doing so would be a fair amount (by index standards) of overhead and hassle. Plus they'd get endless complaints any time the S&P500 was outperforming the "S&P 499" that they were using. Plus they'd put themselves in the crosshairs of a whole range of activists who wanted them to switch to an "S&P 498", or ...497, or ... - by excluding various other companies the activists didn't like.
> If they're forced to passively hold...
Their responsibility is managing their pension funds in the interests of their state, and their current & future retirees. Not pious adherence to passive indexing canon. Their calculus here might be to throw a small bone to the anti-Musk activists who are currently bothering them, while acquiring some "we tried!" butt-coverage for whenever Musk really goes off the rails. (And, obviously, trying to discourage other companies from using such control structures.)