My question is simple: in which situations would OpenWRT hit the maximum 27 MB RAM available in the system ? I ask this because the articles state that the memory limitation is worse than the storage limitation, but I can't see this limitation while using the router (and this invalidates the articles arguments).
Typically operations that cause memory peaks due to some large file handling/parsing. E.g. Adblock block list handling, opkg package list handling, etc.
You are already doing the main mitigation by running an ancient deprecated OpenWrt version, which has smaller memory footprint due to the old kernel. That is of course unsecure, but enables you to use the router.