This website feels outdated, if the idea is to use the system fonts you might as well be using system-ui (although that can come with its own set of can of worms if you care about i18n). Specially because it's using -apple-system which is literally the same thing as system-ui nowadays (Apple was the one to propose and push for system-ui).

The font stacks themselves seem solid though. Specially for serif and mono where of course system doesn't cut and ui-monospace is only supported on Apple devices (similar to system-ui but for monospace).