After a little digging, I've come up with an idea, that the provisioning software is using another version of the library, probably being distributed alongside it.
So, after searching in my file system, I found a libreadline.so.8 binary under /opt/nxp/MCUX_Provi_v8/bin/_internal/. After removing it, the tool could load the system installed libreadline.so.8.2 and the ext_memory_setup.sh script worked as intended.
Seens that a litte version bump in the provisiong tool libreadline to 8.2 would resolve the problem for linux users.