That's just how a call option works. I don't know the specifics of custodianship around an executed option and what collateral the buyer's broker is responsible

That's just how a call option works. I don't know the specifics of custodianship around an executed option and what collateral the buyer's broker is responsible for upon execution, but it's probably equivalent to buying 100 shares. With respect to it being possible -- if RH denied options holders the right to execute they'd be prosecuted and fined an enormous amount. There is quite literally a contract that says the buyer has the right to acquire shares in a certain scenario -- the buyer's broker responsibility here is simply to facilitate, and is in no place legally to deny that right.