AFAICT, this fix requires setting both parameters (intel_iommu and iommu) to work.
No, I have absolutely no idea why this works, but it involved a LOT of trial and error.
After installation,
# vi /etc/default/grub
Change
======================
FROM
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
TO:
GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on iommu=pt"
======================
# update-grub
# reboot
or
# shutdown -r now
And IPMI works again after the reboot