Yes, there is quite a bit of difference between the two. I didn't spot it initially, but the kernel has been rebuilt (but supposedly from the same version), it's not just a repackage with login disabled. I have tried two different ttl serial adapters (funnily enough one of which does fail the loopback test now that I try it, but the other is fine), same result with both.
I note in your firewall rules that telnet is allowed to pass through if the packets are MARKed with 0x102001, but I can't see anywhere that these are getting marked. I suspect that the default iptables -L isn't showing all the tables. Any chance of a iptables-save output, or looking at the contents of /bin/sec_init.sh or /etc/wap/sec_init ? These files contain the iptables rules in the extracted rootfs from the latest version (V100R006C00SPC122), but they they don't have as many rules in them as the deployed BT version you have on yours (and specifically don't include the marked packet filters). On the other hand, I don;t see any sign of a device at 192.168.100.1 at all, but I'll switch to testing from Linux when I get chance because I don't trust networking in Windows 10.