A complete guess, but I would deduce the opposite, as I/O boats have a hole cut out below the water liner, where outboard bolts are above it. Smaller outboards may have more issues if people clamp them down too tightly and it splits the fiberglass
No matter what boat IO or outboard its all depends on water and air. Most boat owners think nothing of drilling a hole into a boat and putting a bolt or a screw in the hole, but never seal it properly.
It's just a little leak and heck that why I have a bilge pump !
You have to remember it takes water and air to rot wood. Wood that is submerged all the time without oxygen will not rot. It's the combo of the two.
I see boaters all the time drill into a sealed fiberglass wood structure and not seal the the hole.
I would say you would see more IO with rot only because like lowkee said holes under water and not enough maintance and a leak develops. The manufactures go through all the time and trouble to build a decent boat and then drill holes in them and use cheap sealers that dry up and leak in 5 years and presto ROT.
