I admit it's been a long time since I've done any advanced iptables stuff, so my answer was a bit hasty, and indeed completely wrong (that rule matches natted packets, it does not jump to the nat table). Looking more closely at this, I still doubt that this would be an issue.
My theory is that, to hit a LAN device with an unroutable IP (such as 192.168.1.100), someone would have to be connected directly on your WAN port to submit the crafted packet with a source IP being his IP, and a target IP being the LAN target. Otherwise, such a packet would never even reach your device - your ISP can't route packets to you if they have a destination address that is a non-routable IP. The only packets routed to you have a destination IP that's your WAN IP (unless you actually have a routed block). That ends up on your router, at which point it will go through the whole iptable system to process the packet, forwarding it to a LAN PC only if there is a DNAT rule for it.
Caveat: someone with more advanced network knowledge could still prove me wrong - I admit to not being a networking engineer myself. If so, I'd be interested to read the correct explanation.
As an aside, I dug out a very informative flowchart showing how the various chains are related to one another in iptables:
EDIT: one scenario I just thought of where this could become an issue is if instead of fronting the Internet your router was fronting another private network segment. Then, it could become imaginable for someone to submit a packet with a non-routable IP directly to your router.