How light? As you mentioned, an SUL knife isn't going to be much help in a 72 hour survival scenario, but there is a whole range of cutting tools from a single edge razor blade on out to an ax or the "sharpened pry bar" type of knife. I opt for a 3.5" quality folder like a Benchmade Griptilian that is 3.8oz. If I want more cutting power in a survival situation and light weight, I would go for a light saw rather than a big knife or ax: 3.5oz vs a couple pounds.
The gist of it is that I want the lightest gear that will function to my needs. In a survival scenario, shelter, fire, navigation, hydration, signaling and first aid come to mind.
We could kick first aid kits around and it comes to minor wound care like blisters, splinters and small cuts and then it kind of blossoms to near military sorts of needs like compound fractures, arterial bleeds, puncture wounds, etc. I think most opt to carry small wound care and hope that improvised methods will take care of the big bad stuff. 3-4 ounces will go a long ways.
Good lighting can be had in light enough forms. The best compasses aren't that heavy. Signaling devices like whistles and mirrors don't have to be heavy. Fire starting methods are light, even if redundant forms are carried. Hydration is simple enough with light containers and chlorine dioxide tablets are super light. I covered tools for making shelters from natural sources. Something like a poncho tarp and light bivy sack will make emergency shelter that is carried rather than found/improvised.
I could go lighter on a knife, but we're talking 2-3 ounces between my "heavy" knife and a lighter folder. I could take a button compass vs my boat anchor 2.6oz compass with mirror. A whistle that is loud enough to functionally deafen the user is all of 0.2oz. My headlamps range from 1oz to nearly 7, but a 3oz one is very functional (I'm not risking my life on a coin cell LED light). My firesteel racks up 0.4oz. My match case (with redundant button compass) is 1.4oz loaded with matches. A silnylon poncho is 7oz--- I could opt for a lighter Cuben model, but the question there is really expense rather than weight. A space blanket bivy is 3.4oz. The cobra-braid paracord hatband on my Tilly provides a source of cordage.
I think that it comes down to the shear number of items more than the weight of each. In a survival scenario, I want stuff that works AND a selection of items that will allow me to improvise to handle a whole range of unknowns.
Using good ultralight principles, we should seek out items that would actually be used, have multiple uses, and be of high performance for the weight. That kind of gets into a whole pile of gray areas on what is really going to be used. In a perfect world, you never use your survival stuff, so I say use some common sense, try to fit your CYA items to the terrain and season, and don't try to cover every possible scenario. Seek out gear that gives good performance, use your head, do the best you can.
DON'T GET STUCK IN THE FIRST PLACE, which is very light indeed :)