mdev: can't create /proc/sys/kernel/hotplug: nonexistent directory (3.11.0_rc2 aarch64)

I wrote http://dl-cdn.alpinelinux.org/alpine/v3.11/releases/aarch64/alpine-rpi-3.11.0_rc2-aarch64.tar.gz to SD card and booted on Raspberry Pi 3B+.

On boot I see this warning:

* Starting busybox mdev ...
/lib/rc/sh/openrc-run.sh: line 21: can't create /proc/sys/kernel/hotplug: nonexistent directory

I note also that although my devices is 3B+, the login prompt says:

Welcome to Alpine Linux 3.11
Kernel 5.4.3-0-rpi4 on an aarch64 (/dev/tty1)

(rpi4 instead of rpi?)