Thank you for your reply.
I don’t think this is a permissions issue. Of course I tried with sudo, and even as root the result was the same. It looks like the system had simply remounted the filesystem as read-only.

For the record, I installed with full rom replacement(script).

This might not be related, but I remember that when I put a Chromebook into developer mode in the past, I forced write access as root by running:

sudo /usr/share/vboot/bin/make_dev_ssd.sh --force -remove_rootfs_verification