Yeah, even if syslinux 6.x “falls back” when E820/E801 aren’t there, AH=88h is just a number, not a hole aware map. It can’t describe ISA/ROM/reserved regions, so the loader still might not be able to build something it trusts.
If you want to prove it yourself, stick a log right around syslinux_memmap_find() and dump the memmap entries it thinks it has on that BIOS.
Its interesting to notice that syslinux 6.03 had a fix for your same issue:
"Fix hand over to kernel:
"Booting kernel failed: Invalid argument" error."
https://www.linuxquestions.org/questions/slackware-14/%5Bann...Based on the POST strings of your motherboard versus mine (https://theretroweb.com/motherboards/s/fic-486-jal-rev-c), we both have an AMI BIOS so I might be able to run a similar test for my board. You're right that syslinux 6.x should fall back if E820 doesn't work but that hasn't been my experience on my motherboard hence the reversion to 4.x, I can reliably boot with 4-16MB of RAM.