What he probably has access to is people who review his feed and replies and curate that for him. So he's not responding to tweets he's found himself; but rather to tweets that have been highlighted for him to respond to (if he so desires). Heck, his team might actually include possible response ideas.

The fallacy is thinking Elon uses Twitter like any other person uses Twitter. Sitting on his device, manually scrolling through it and replying sequentially. He's probably outsourced much of this to his media teams or other people he trusts to provide relevant material to engage with.