@slh: Thank you. I think this did change at some point in the past.
RESOLVED:
The basic issue is that for current Hornet boards (Alfa AP121) there is no "factory" image.
Up to Chaos Calmer (15) there were images for the 8MB and the 16MB version provided.
In addition to that there's a big warning in AP121/Hornet articles about " NOT RECOMMENDED for future use" and also "OpenWrt support for this device will end after 2019." which is completely misleading!
First of all there is a 4/32 version mentioned which I actually never faced. And in pages from 2015 there is only the 8/32 mentioned - maybe 4/32 was just a prototype.
There are even more doc parts which are not uptodate and even images seem to have been "vanished" since migration from old wiki.
Currently only 16/64 boards are available and these are supported but there is no factory image provided anymore.
You'll need to upgrade manually to get the benefits from the 16MB like e.g. use openwrt-19.07.7-ar71xx-generic-hornet-ub-x2-squashfs-sysupgrade.bin as kernel image to flash and openwrt-19.07.7-ar71xx-generic-ap121-16M-squashfs-sysupgrade.bin as rootfs image.