I'm in an async company and team, we practice scrum too. Half the team in the US, half in Europe.
First of all, the standup isn't daily, it's twice a week. People still miss it for reasons (kids, doctors appointments, ...), and it's just... accepted. There's a general culture of trust.
Other than that, Slack threads go on twice as verbose and long as in other places I've worked. There's a general deluge of pull requests going around certain times of the day and they're always reviewed within an acceptable time frame. In between delivering PR's you simply pick up other work, we're all seniors that know how to work independently, I'm not sure if you could achieve the same thing with a bunch of juniors mixed in unless they have the right attitude as well.